diff --git a/backend/apps/api/src/trips/trip-invitations.controller.spec.ts b/backend/apps/api/src/trips/trip-invitations.controller.spec.ts
new file mode 100644
index 0000000..5ff7bf2
--- /dev/null
+++ b/backend/apps/api/src/trips/trip-invitations.controller.spec.ts
@@ -0,0 +1,78 @@
+import { TripInvitationsController } from './trip-invitations.controller';
+import type { AuthenticatedUser } from '../../../../libs/auth/src';
+
+describe('TripInvitationsController', () => {
+  const currentUser: AuthenticatedUser = {
+    id: 'owner-1',
+    externalSubjectId: 'sub-1',
+    displayName: 'Owner',
+    email: 'owner@example.com',
+  };
+
+  it('POST /trips/:tripId/invitations creates an invitation and returns the raw token', async () => {
+    const result = {
+      invitation: { id: 'inv-1', tripId: 't1', email: 'friend@example.com' },
+      rawToken: 'raw-token',
+    };
+    const invitationsService = {
+      createInvitation: jest.fn().mockResolvedValue(result),
+    };
+    const controller = new TripInvitationsController(
+      invitationsService as never,
+    );
+
+    await expect(
+      controller.create('t1', currentUser, { email: 'friend@example.com' }),
+    ).resolves.toEqual(result);
+    expect(invitationsService.createInvitation).toHaveBeenCalledWith(
+      't1',
+      'owner-1',
+      'friend@example.com',
+    );
+  });
+
+  it('GET /trips/:tripId/invitations lists invitations', async () => {
+    const invitationsService = {
+      listInvitations: jest.fn().mockResolvedValue([]),
+    };
+    const controller = new TripInvitationsController(
+      invitationsService as never,
+    );
+
+    await expect(controller.list('t1')).resolves.toEqual([]);
+    expect(invitationsService.listInvitations).toHaveBeenCalledWith('t1');
+  });
+
+  it('DELETE /trips/:tripId/invitations/:invitationId removes an invitation', async () => {
+    const invitationsService = {
+      removeInvitation: jest.fn().mockResolvedValue(undefined),
+    };
+    const controller = new TripInvitationsController(
+      invitationsService as never,
+    );
+
+    await controller.remove('t1', 'inv-1');
+    expect(invitationsService.removeInvitation).toHaveBeenCalledWith(
+      't1',
+      'inv-1',
+    );
+  });
+
+  it('POST /invitations/:token/accept accepts the invitation for the current user', async () => {
+    const member = { id: 'm1', tripId: 't1', userId: 'owner-1' };
+    const invitationsService = {
+      acceptInvitation: jest.fn().mockResolvedValue(member),
+    };
+    const controller = new TripInvitationsController(
+      invitationsService as never,
+    );
+
+    await expect(controller.accept('raw-token', currentUser)).resolves.toEqual(
+      member,
+    );
+    expect(invitationsService.acceptInvitation).toHaveBeenCalledWith(
+      'raw-token',
+      'owner-1',
+    );
+  });
+});
diff --git a/backend/apps/api/src/trips/trip-invitations.controller.ts b/backend/apps/api/src/trips/trip-invitations.controller.ts
new file mode 100644
index 0000000..f286197
--- /dev/null
+++ b/backend/apps/api/src/trips/trip-invitations.controller.ts
@@ -0,0 +1,68 @@
+import {
+  Body,
+  Controller,
+  Delete,
+  Get,
+  Param,
+  Post,
+  UseGuards,
+} from '@nestjs/common';
+import { OidcAuthGuard } from '../../../../libs/auth/src';
+import type { AuthenticatedUser } from '../../../../libs/auth/src';
+import {
+  TripInvitationsService,
+  TripMembershipGuard,
+  TripRoles,
+} from '../../../../libs/trips/src';
+import type { TripInvitation, TripMember } from '../../../../libs/trips/src';
+import { CurrentUser } from '../auth/current-user.decorator';
+
+interface CreateTripInvitationDto {
+  email: string;
+}
+
+@Controller()
+export class TripInvitationsController {
+  constructor(private readonly invitationsService: TripInvitationsService) {}
+
+  @Post('trips/:tripId/invitations')
+  @UseGuards(OidcAuthGuard, TripMembershipGuard)
+  @TripRoles('OWNER')
+  create(
+    @Param('tripId') tripId: string,
+    @CurrentUser() currentUser: AuthenticatedUser,
+    @Body() dto: CreateTripInvitationDto,
+  ): Promise<{ invitation: TripInvitation; rawToken: string }> {
+    return this.invitationsService.createInvitation(
+      tripId,
+      currentUser.id,
+      dto.email,
+    );
+  }
+
+  @Get('trips/:tripId/invitations')
+  @UseGuards(OidcAuthGuard, TripMembershipGuard)
+  @TripRoles('OWNER')
+  list(@Param('tripId') tripId: string): Promise<TripInvitation[]> {
+    return this.invitationsService.listInvitations(tripId);
+  }
+
+  @Delete('trips/:tripId/invitations/:invitationId')
+  @UseGuards(OidcAuthGuard, TripMembershipGuard)
+  @TripRoles('OWNER')
+  remove(
+    @Param('tripId') tripId: string,
+    @Param('invitationId') invitationId: string,
+  ): Promise<void> {
+    return this.invitationsService.removeInvitation(tripId, invitationId);
+  }
+
+  @Post('invitations/:token/accept')
+  @UseGuards(OidcAuthGuard)
+  accept(
+    @Param('token') token: string,
+    @CurrentUser() currentUser: AuthenticatedUser,
+  ): Promise<TripMember> {
+    return this.invitationsService.acceptInvitation(token, currentUser.id);
+  }
+}
diff --git a/backend/apps/api/src/trips/trips.module.ts b/backend/apps/api/src/trips/trips.module.ts
index cf95983..0d43e53 100644
--- a/backend/apps/api/src/trips/trips.module.ts
+++ b/backend/apps/api/src/trips/trips.module.ts
@@ -3,9 +3,14 @@ import { AuthModule } from '../../../../libs/auth/src';
 import { TripsLibModule } from '../../../../libs/trips/src';
 import { TripsController } from './trips.controller';
 import { TripMembersController } from './trip-members.controller';
+import { TripInvitationsController } from './trip-invitations.controller';
 
 @Module({
   imports: [AuthModule, TripsLibModule],
-  controllers: [TripsController, TripMembersController],
+  controllers: [
+    TripsController,
+    TripMembersController,
+    TripInvitationsController,
+  ],
 })
 export class TripsApiModule {}
diff --git a/backend/libs/trips/src/index.ts b/backend/libs/trips/src/index.ts
index d04ef17..9b607d7 100644
--- a/backend/libs/trips/src/index.ts
+++ b/backend/libs/trips/src/index.ts
@@ -3,5 +3,6 @@ export * from './trips.service';
 export * from './trip-settings.service';
 export * from './trip-members.service';
 export * from './trip-membership.guard';
+export * from './trip-invitations.service';
 export * from './trip-roles.decorator';
 export * from './trips.module';
diff --git a/backend/libs/trips/src/trip-invitations.repository.ts b/backend/libs/trips/src/trip-invitations.repository.ts
new file mode 100644
index 0000000..d9921a8
--- /dev/null
+++ b/backend/libs/trips/src/trip-invitations.repository.ts
@@ -0,0 +1,91 @@
+import { Inject, Injectable } from '@nestjs/common';
+import type { Kysely } from 'kysely';
+import { KYSELY_DB } from '../../database/src';
+import type { Database } from '../../database/src';
+import type { CreateTripInvitationFields, TripInvitation } from './trip.types';
+
+function toTripInvitation(row: {
+  id: string;
+  trip_id: string;
+  email: string;
+  invited_by_user_id: string;
+  token_hash: string;
+  expires_at: Date;
+  accepted_at: Date | null;
+  created_at: Date;
+  updated_at: Date;
+}): TripInvitation {
+  return {
+    id: row.id,
+    tripId: row.trip_id,
+    email: row.email,
+    invitedByUserId: row.invited_by_user_id,
+    tokenHash: row.token_hash,
+    expiresAt: row.expires_at,
+    acceptedAt: row.accepted_at,
+    createdAt: row.created_at,
+    updatedAt: row.updated_at,
+  };
+}
+
+@Injectable()
+export class TripInvitationsRepository {
+  constructor(@Inject(KYSELY_DB) private readonly db: Kysely<Database>) {}
+
+  async create(
+    tripId: string,
+    fields: CreateTripInvitationFields,
+  ): Promise<TripInvitation> {
+    const row = await this.db
+      .insertInto('trip_invitations')
+      .values({
+        trip_id: tripId,
+        email: fields.email,
+        invited_by_user_id: fields.invitedByUserId,
+        token_hash: fields.tokenHash,
+        expires_at: fields.expiresAt.toISOString(),
+      })
+      .returningAll()
+      .executeTakeFirstOrThrow();
+    return toTripInvitation(row);
+  }
+
+  async findByTokenHash(
+    tokenHash: string,
+  ): Promise<TripInvitation | undefined> {
+    const row = await this.db
+      .selectFrom('trip_invitations')
+      .selectAll()
+      .where('token_hash', '=', tokenHash)
+      .executeTakeFirst();
+    return row ? toTripInvitation(row) : undefined;
+  }
+
+  async listByTrip(tripId: string): Promise<TripInvitation[]> {
+    const rows = await this.db
+      .selectFrom('trip_invitations')
+      .selectAll()
+      .where('trip_id', '=', tripId)
+      .execute();
+    return rows.map(toTripInvitation);
+  }
+
+  async markAccepted(invitationId: string): Promise<void> {
+    await this.db
+      .updateTable('trip_invitations')
+      .set({
+        accepted_at: new Date().toISOString(),
+        updated_at: new Date().toISOString(),
+      })
+      .where('id', '=', invitationId)
+      .execute();
+  }
+
+  async remove(tripId: string, invitationId: string): Promise<void> {
+    await this.db
+      .deleteFrom('trip_invitations')
+      .where('trip_id', '=', tripId)
+      .where('id', '=', invitationId)
+      .execute();
+  }
+}
diff --git a/backend/libs/trips/src/trip-invitations.service.spec.ts b/backend/libs/trips/src/trip-invitations.service.spec.ts
new file mode 100644
index 0000000..93cb518
--- /dev/null
+++ b/backend/libs/trips/src/trip-invitations.service.spec.ts
@@ -0,0 +1,119 @@
+import {
+  BadRequestException,
+  ConflictException,
+  NotFoundException,
+} from '@nestjs/common';
+import { TripInvitationsService } from './trip-invitations.service';
+
+describe('TripInvitationsService.acceptInvitation', () => {
+  const future = new Date(Date.now() + 60_000);
+  const past = new Date(Date.now() - 60_000);
+
+  it('rejects accepting the same invitation twice', async () => {
+    const repo = {
+      findByTokenHash: jest.fn().mockResolvedValue({
+        id: 'inv-1',
+        tripId: 't1',
+        expiresAt: future,
+        acceptedAt: new Date(),
+      }),
+    };
+    const members = { upsertActiveMember: jest.fn() };
+    const service = new TripInvitationsService(repo as never, members as never);
+
+    await expect(
+      service.acceptInvitation('raw-token', 'user-3'),
+    ).rejects.toThrow(ConflictException);
+    expect(members.upsertActiveMember).not.toHaveBeenCalled();
+  });
+
+  it('rejects an expired invitation', async () => {
+    const repo = {
+      findByTokenHash: jest.fn().mockResolvedValue({
+        id: 'inv-1',
+        tripId: 't1',
+        expiresAt: past,
+        acceptedAt: null,
+      }),
+    };
+    const members = { upsertActiveMember: jest.fn() };
+    const service = new TripInvitationsService(repo as never, members as never);
+
+    await expect(
+      service.acceptInvitation('raw-token', 'user-2'),
+    ).rejects.toThrow(BadRequestException);
+    expect(members.upsertActiveMember).not.toHaveBeenCalled();
+  });
+
+  it('rejects an unknown token without leaking whether the trip exists', async () => {
+    const repo = { findByTokenHash: jest.fn().mockResolvedValue(undefined) };
+    const members = { upsertActiveMember: jest.fn() };
+    const service = new TripInvitationsService(repo as never, members as never);
+
+    await expect(
+      service.acceptInvitation('does-not-exist', 'user-2'),
+    ).rejects.toThrow(NotFoundException);
+  });
+
+  it('accepts a fresh, unexpired invitation and activates trip membership', async () => {
+    const repo = {
+      findByTokenHash: jest.fn().mockResolvedValue({
+        id: 'inv-1',
+        tripId: 't1',
+        expiresAt: future,
+        acceptedAt: null,
+      }),
+      markAccepted: jest.fn().mockResolvedValue(undefined),
+    };
+    const members = {
+      upsertActiveMember: jest
+        .fn()
+        .mockResolvedValue({ id: 'm1', tripId: 't1', userId: 'user-2' }),
+    };
+    const service = new TripInvitationsService(repo as never, members as never);
+
+    const member = await service.acceptInvitation('raw-token', 'user-2');
+
+    expect(repo.markAccepted).toHaveBeenCalledWith('inv-1');
+    expect(members.upsertActiveMember).toHaveBeenCalledWith(
+      't1',
+      'user-2',
+      'MEMBER',
+    );
+    expect(member).toEqual({ id: 'm1', tripId: 't1', userId: 'user-2' });
+  });
+});
+
+describe('TripInvitationsService.createInvitation', () => {
+  it('stores only a hash of the generated token and returns the raw token once', async () => {
+    const repo = {
+      create: jest
+        .fn()
+        .mockImplementation((tripId: string, fields: Record<string, unknown>) =>
+          Promise.resolve({ id: 'inv-1', tripId, ...fields }),
+        ),
+    };
+    const members = { upsertActiveMember: jest.fn() };
+    const service = new TripInvitationsService(repo as never, members as never);
+
+    const result = await service.createInvitation(
+      't1',
+      'owner-1',
+      'friend@example.com',
+    );
+
+    expect(result.rawToken).toEqual(expect.any(String));
+    expect(repo.create).toHaveBeenCalledWith(
+      't1',
+      expect.objectContaining({
+        email: 'friend@example.com',
+        invitedByUserId: 'owner-1',
+      }),
+    );
+    const [, fields] = repo.create.mock.calls[0] as [
+      string,
+      { tokenHash: string },
+    ];
+    expect(fields.tokenHash).not.toBe(result.rawToken);
+  });
+});
diff --git a/backend/libs/trips/src/trip-invitations.service.ts b/backend/libs/trips/src/trip-invitations.service.ts
new file mode 100644
index 0000000..c29cb5f
--- /dev/null
+++ b/backend/libs/trips/src/trip-invitations.service.ts
@@ -0,0 +1,69 @@
+import { randomBytes, createHash } from 'node:crypto';
+import {
+  BadRequestException,
+  ConflictException,
+  Injectable,
+  NotFoundException,
+} from '@nestjs/common';
+import { TripInvitationsRepository } from './trip-invitations.repository';
+import { TripMembersRepository } from './trip-members.repository';
+import type { TripInvitation, TripMember } from './trip.types';
+
+const DEFAULT_TTL_MS = 7 * 24 * 60 * 60 * 1000;
+
+function hashToken(rawToken: string): string {
+  return createHash('sha256').update(rawToken).digest('hex');
+}
+
+@Injectable()
+export class TripInvitationsService {
+  constructor(
+    private readonly invitationsRepository: TripInvitationsRepository,
+    private readonly membersRepository: TripMembersRepository,
+  ) {}
+
+  async createInvitation(
+    tripId: string,
+    invitedByUserId: string,
+    email: string,
+    ttlMs = DEFAULT_TTL_MS,
+  ): Promise<{ invitation: TripInvitation; rawToken: string }> {
+    const rawToken = randomBytes(32).toString('base64url');
+    const invitation = await this.invitationsRepository.create(tripId, {
+      email,
+      invitedByUserId,
+      tokenHash: hashToken(rawToken),
+      expiresAt: new Date(Date.now() + ttlMs),
+    });
+    return { invitation, rawToken };
+  }
+
+  listInvitations(tripId: string): Promise<TripInvitation[]> {
+    return this.invitationsRepository.listByTrip(tripId);
+  }
+
+  removeInvitation(tripId: string, invitationId: string): Promise<void> {
+    return this.invitationsRepository.remove(tripId, invitationId);
+  }
+
+  async acceptInvitation(
+    rawToken: string,
+    acceptingUserId: string,
+  ): Promise<TripMember> {
+    const invitation = await this.invitationsRepository.findByTokenHash(
+      hashToken(rawToken),
+    );
+    if (!invitation) throw new NotFoundException('Invitation not found');
+    if (invitation.acceptedAt)
+      throw new ConflictException('Invitation has already been accepted');
+    if (invitation.expiresAt.getTime() < Date.now())
+      throw new BadRequestException('Invitation has expired');
+
+    await this.invitationsRepository.markAccepted(invitation.id);
+    return this.membersRepository.upsertActiveMember(
+      invitation.tripId,
+      acceptingUserId,
+      'MEMBER',
+    );
+  }
+}
diff --git a/backend/libs/trips/src/trip.types.ts b/backend/libs/trips/src/trip.types.ts
index b21014e..06d73f4 100644
--- a/backend/libs/trips/src/trip.types.ts
+++ b/backend/libs/trips/src/trip.types.ts
@@ -78,6 +78,25 @@ export interface TripMember {
   updatedAt: Date;
 }
 
+export interface TripInvitation {
+  id: string;
+  tripId: string;
+  email: string;
+  invitedByUserId: string;
+  tokenHash: string;
+  expiresAt: Date;
+  acceptedAt: Date | null;
+  createdAt: Date;
+  updatedAt: Date;
+}
+
+export interface CreateTripInvitationFields {
+  email: string;
+  invitedByUserId: string;
+  tokenHash: string;
+  expiresAt: Date;
+}
+
 export const DEFAULT_TRIP_SETTINGS: Omit<TripSettings, 'tripId'> = {
   webResearchEnabled: false,
   periodicAgentReviewEnabled: false,
diff --git a/backend/libs/trips/src/trips.module.ts b/backend/libs/trips/src/trips.module.ts
index cdcb8b3..ea6f6da 100644
--- a/backend/libs/trips/src/trips.module.ts
+++ b/backend/libs/trips/src/trips.module.ts
@@ -7,6 +7,8 @@ import { TripSettingsService } from './trip-settings.service';
 import { TripMembersRepository } from './trip-members.repository';
 import { TripMembersService } from './trip-members.service';
 import { TripMembershipGuard } from './trip-membership.guard';
+import { TripInvitationsRepository } from './trip-invitations.repository';
+import { TripInvitationsService } from './trip-invitations.service';
 
 @Module({
   imports: [DatabaseModule],
@@ -18,12 +20,15 @@ import { TripMembershipGuard } from './trip-membership.guard';
     TripMembersRepository,
     TripMembersService,
     TripMembershipGuard,
+    TripInvitationsRepository,
+    TripInvitationsService,
   ],
   exports: [
     TripsService,
     TripSettingsService,
     TripMembersService,
     TripMembershipGuard,
+    TripInvitationsService,
   ],
 })
 export class TripsLibModule {}