diff --git a/README.md b/README.md
index 2a6519f..8baf140 100644
--- a/README.md
+++ b/README.md
@@ -32,6 +32,7 @@ This repository contains various examples on how to setup **io.Manager**
 # Other
 
 - [custom-endpoints](./custom-endpoints) - An example that demonstrates how to implement custom endpoints to **io.Manager**.
+- [custom-groups-service](./custom-groups-service) - An example that demonstrates how to provide a custom Groups service to **io.Manager**.
 - [node-esm](./node-esm) - An example that demonstrates how to setup a **io.Manager** in a Node.js native ESM project.
 - [node-commonjs](./node-commonjs) - An example that demonstrates how to setup a **io.Manager** in a Node.js native CommonJS project.
 - [custom-logging-config](./custom-logging-config) - An example that demonstrates how to pass custom logging configuration to **io.Manager**.
diff --git a/custom-groups-service/.gitignore b/custom-groups-service/.gitignore
new file mode 100644
index 0000000..633c49a
--- /dev/null
+++ b/custom-groups-service/.gitignore
@@ -0,0 +1,4 @@
+**/node_modules
+
+**/dist
+**/logs
diff --git a/custom-groups-service/README.md b/custom-groups-service/README.md
new file mode 100644
index 0000000..c02ff8a
--- /dev/null
+++ b/custom-groups-service/README.md
@@ -0,0 +1,43 @@
+# Introduction
+
+An example that demonstrates how to provide a custom Groups service to **io.Manager**.
+
+# Custom Groups service
+
+By default **io.Manager** stores groups and resolves the groups a user belongs to using its internal Groups Service implementation. Providing a custom Groups service through the `groups_service` configuration property replaces that with your own implementation.
+
+A custom Groups service implements the `GroupsService` interface exported from `@interopio/manager`. Its `getSupportedFeatures()` method declares which operations the implementation supports - **io.Manager** invokes a group management operation only when the corresponding capability flag is `true`, so an implementation that only reads groups can report `false` for the write operations and leave those methods as stubs.
+
+This example implements [MyGroupsService](./src/MyGroupsService.ts) as a thin adapter over the in-memory stores in [data.ts](./src/data.ts) that stand in for an external system.
+
+For an example that combines a custom Groups service with a custom authenticator, see the [auth-custom](../auth-custom) example.
+
+# Prerequisites
+
+### Database
+
+io.Manager requires a database to connect to - this example uses MongoDB, but you can use any other of the supported databases. You will need to either have a local instance or setup a remote database to connect to. For more information visit our Documentation page on the subject: https://docs.interop.io/manager/databases/overview/index.html
+
+### License
+
+**io.Manager** requires a license key to operate. To acquire a license key, contact us at `sales@interop.io`.
+
+# How to run
+
+- Install npm packages
+
+```sh
+
+npm install
+
+npm audit fix
+
+```
+
+- Start the server
+
+```sh
+
+npm run start
+
+```
diff --git a/custom-groups-service/package.json b/custom-groups-service/package.json
new file mode 100644
index 0000000..2e3f091
--- /dev/null
+++ b/custom-groups-service/package.json
@@ -0,0 +1,17 @@
+{
+  "name": "custom-groups-service",
+  "version": "0.0.1",
+  "private": true,
+  "type": "commonjs",
+  "scripts": {
+    "build": "npm run delete ./dist && tsc",
+    "delete": "node -e \"const fs = require('fs'); const path = process.argv[1]; if (!path) process.exit(1); if (fs.existsSync(path)) fs.rmSync(path, { recursive: true, force: true });\"",
+    "start": "npm run build && node ./dist/index.js"
+  },
+  "dependencies": {
+    "@interopio/manager": "^4.0.0"
+  },
+  "devDependencies": {
+    "typescript": "^5.9.3"
+  }
+}
diff --git a/custom-groups-service/src/MyGroupsService.ts b/custom-groups-service/src/MyGroupsService.ts
new file mode 100644
index 0000000..12ead77
--- /dev/null
+++ b/custom-groups-service/src/MyGroupsService.ts
@@ -0,0 +1,90 @@
+import type {
+  AuditBuilder,
+  DataRequest,
+  Group,
+  GroupDataResult,
+  GroupsFeatures,
+  GroupsService,
+  User,
+} from '@interopio/manager';
+
+import { groups, users } from './data.js';
+
+export class MyGroupsService implements GroupsService {
+  // Declares which operations this implementation supports.
+  public getSupportedFeatures(): GroupsFeatures {
+    return {
+      canGetUserGroups: true,
+      canGetAllGroups: true,
+      canGetGroup: true,
+      canAddGroup: true,
+      canUpdateGroup: true,
+      canAddOrUpdateGroup: true,
+      canRemoveGroup: true,
+      canAddUserToGroup: true,
+      canRemoveUserFromGroup: true,
+    };
+  }
+
+  // ⚠️ io.Manager authorizes each request against the groups returned here - apart from the
+  // admin rights granted through `auth_exclusive_users`, this decides what a user can access.
+  public async getUserGroups(user: string | User): Promise<string[]> {
+    const id = typeof user === 'string' ? user : user.id;
+
+    return users.getGroups(id);
+  }
+
+  public async getAllGroups(_request?: DataRequest): Promise<GroupDataResult> {
+    const items = groups.getAll();
+
+    return {
+      items,
+      total: items.length,
+    };
+  }
+
+  public async getGroup(name: string): Promise<Group | undefined> {
+    return groups.get(name);
+  }
+
+  public async addGroup(group: Group, _audit: AuditBuilder): Promise<Group> {
+    return groups.add(group);
+  }
+
+  public async updateGroup(group: Group, _audit: AuditBuilder): Promise<Group> {
+    return groups.update(group);
+  }
+
+  public async addOrUpdateGroup(
+    group: Group,
+    _audit: AuditBuilder
+  ): Promise<Group> {
+    return groups.addOrUpdate(group);
+  }
+
+  public async removeGroup(name: string, _audit: AuditBuilder): Promise<void> {
+    groups.remove(name);
+    users.removeGroupFromAll(name);
+  }
+
+  public async addUserToGroups(
+    user: string,
+    groups: string[],
+    _audit: AuditBuilder
+  ): Promise<void> {
+    users.addToGroups(user, groups);
+  }
+
+  public async removeUserFromGroups(
+    user: string,
+    groups: string[],
+    _audit: AuditBuilder
+  ): Promise<void> {
+    users.removeFromGroups(user, groups);
+  }
+
+  public async removeAll(_audit: AuditBuilder): Promise<void> {
+    groups.clear();
+    users.clearGroups();
+  }
+}
diff --git a/custom-groups-service/src/data.ts b/custom-groups-service/src/data.ts
new file mode 100644
index 0000000..dce21a4
--- /dev/null
+++ b/custom-groups-service/src/data.ts
@@ -0,0 +1,142 @@
+import type { Group, User } from '@interopio/manager';
+
+export const GROUP_SERVER_ADMIN = 'GLUE42_SERVER_ADMIN';
+export const GROUP_FRONT_OFFICE = 'Front Office';
+export const GROUP_TRADER = 'Trader';
+
+export class InMemoryGroupsStore {
+  private readonly groups: Group[] = [
+    {
+      name: GROUP_FRONT_OFFICE,
+    },
+    {
+      name: GROUP_TRADER,
+    },
+  ];
+
+  public getAll(): Group[] {
+    return [...this.groups];
+  }
+
+  public get(name: string): Group | undefined {
+    return this.groups.find((group) => group.name === name);
+  }
+
+  public add(group: Group): Group {
+    if (!this.groups.some((existing) => existing.name === group.name)) {
+      this.groups.push(group);
+    }
+
+    return group;
+  }
+
+  public update(group: Group): Group {
+    const index = this.groups.findIndex(
+      (existing) => existing.name === group.name
+    );
+
+    if (index !== -1) {
+      this.groups[index] = group;
+    }
+
+    return group;
+  }
+
+  public addOrUpdate(group: Group): Group {
+    const index = this.groups.findIndex(
+      (existing) => existing.name === group.name
+    );
+
+    if (index !== -1) {
+      this.groups[index] = group;
+    } else {
+      this.groups.push(group);
+    }
+
+    return group;
+  }
+
+  public remove(name: string): void {
+    const index = this.groups.findIndex((group) => group.name === name);
+
+    if (index !== -1) {
+      this.groups.splice(index, 1);
+    }
+  }
+
+  public clear(): void {
+    this.groups.length = 0;
+  }
+}
+
+export class InMemoryUsersStore {
+  private readonly users: User[] = [
+    {
+      id: 'number.one@company.xyz',
+      email: 'number.one@company.xyz',
+      apps: [],
+      groups: [GROUP_FRONT_OFFICE],
+    },
+    {
+      id: 'number.two@company.xyz',
+      email: 'number.two@company.xyz',
+      apps: [],
+      groups: [GROUP_TRADER],
+    },
+    {
+      id: 'admin',
+      email: 'admin@company.xyz',
+      apps: [],
+      groups: [GROUP_SERVER_ADMIN],
+    },
+  ];
+
+  public get(id: string): User | undefined {
+    return this.users.find((user) => user.id === id);
+  }
+
+  public getGroups(id: string): string[] {
+    return [...(this.get(id)?.groups ?? [])];
+  }
+
+  public addToGroups(id: string, groups: string[]): void {
+    const user = this.get(id);
+
+    if (!user) {
+      return;
+    }
+
+    for (const group of groups) {
+      if (!user.groups.includes(group)) {
+        user.groups.push(group);
+      }
+    }
+  }
+
+  public removeFromGroups(id: string, groups: string[]): void {
+    const user = this.get(id);
+
+    if (!user) {
+      return;
+    }
+
+    user.groups = user.groups.filter((group) => !groups.includes(group));
+  }
+
+  // Drops a group from every user that belonged to it.
+  public removeGroupFromAll(name: string): void {
+    for (const user of this.users) {
+      user.groups = user.groups.filter((group) => group !== name);
+    }
+  }
+
+  public clearGroups(): void {
+    for (const user of this.users) {
+      user.groups = [];
+    }
+  }
+}
+
+export const groups = new InMemoryGroupsStore();
+
+export const users = new InMemoryUsersStore();
diff --git a/custom-groups-service/src/index.ts b/custom-groups-service/src/index.ts
new file mode 100644
index 0000000..438d5bd
--- /dev/null
+++ b/custom-groups-service/src/index.ts
@@ -0,0 +1,27 @@
+import { start, type Config } from '@interopio/manager';
+
+import { MyGroupsService } from './MyGroupsService.js';
+
+const config: Config = {
+  name: 'example',
+  port: 4356,
+  base: 'api',
+  // TODO: Contact us at sales@interop.io to acquire a license key.
+  licenseKey: '<YOUR_LICENSE_KEY>',
+  auth_method: 'none',
+  auth_exclusive_users: ['admin'],
+  store: {
+    type: 'mongo',
+    // TODO: Replace this with your own MongoDB connection string.
+    connection:
+      'mongodb://db_user:Password123$@localhost:27017/io_manager?authSource=admin&directConnection=true',
+  },
+  token: {
+    // TODO: Replace this with your secret.
+    secret: '<YOUR_SECRET>',
+  },
+  // Replaces the built-in Groups service with the custom implementation.
+  groups_service: new MyGroupsService(),
+};
+
+start(config);
diff --git a/custom-groups-service/tsconfig.json b/custom-groups-service/tsconfig.json
new file mode 100644
index 0000000..fa4ff1f
--- /dev/null
+++ b/custom-groups-service/tsconfig.json
@@ -0,0 +1,16 @@
+{
+  "compilerOptions": {
+    "target": "ESNext",
+    "lib": ["ESNext"],
+    "module": "node16",
+    "moduleResolution": "node16",
+    "skipLibCheck": true,
+    "sourceMap": true,
+    "declaration": true,
+    "rootDir": "./src",
+    "outDir": "dist",
+    "pretty": true,
+    "strict": true
+  },
+  "include": ["src"]
+}