diff --git a/src/__tests__/mockUserPoolService.ts b/src/__tests__/mockUserPoolService.ts
index e1c8a4b2..56d6690e 100644
--- a/src/__tests__/mockUserPoolService.ts
+++ b/src/__tests__/mockUserPoolService.ts
@@ -6,7 +6,7 @@ export const newMockUserPoolService = (
     Id: "test",
   }
 ): jest.Mocked<UserPoolService> => ({
-  options: config,
+  addUserToGroup: jest.fn(),
   deleteAppClient: jest.fn(),
   deleteGroup: jest.fn(),
   deleteUser: jest.fn(),
@@ -15,6 +15,7 @@ export const newMockUserPoolService = (
   getUserByUsername: jest.fn(),
   listGroups: jest.fn(),
   listUsers: jest.fn(),
+  options: config,
   removeUserFromGroup: jest.fn(),
   saveAppClient: jest.fn(),
   saveGroup: jest.fn(),
diff --git a/src/services/userPoolService.test.ts b/src/services/userPoolService.test.ts
index 3db701f1..380215a4 100644
--- a/src/services/userPoolService.test.ts
+++ b/src/services/userPoolService.test.ts
@@ -693,4 +693,85 @@ describe("User Pool Service", () => {
       expect(userPool.options.MfaConfiguration).toEqual("ON");
     });
   });
+
+  describe("addUserToGroup", () => {
+    it("updates the group's members", async () => {
+      const ds = newMockDataStore();
+      const userPool = new UserPoolServiceImpl(
+        mockClientsDataStore,
+        clock,
+        ds,
+        {
+          Id: "test",
+        }
+      );
+
+      const user = TDB.user();
+      const group = TDB.group();
+
+      await userPool.addUserToGroup(TestContext, group, user);
+
+      expect(ds.set).toHaveBeenCalledWith(
+        TestContext,
+        ["Groups", group.GroupName],
+        {
+          ...group,
+          LastModifiedDate: clock.get(),
+          members: [user.Username],
+        }
+      );
+    });
+
+    it("only adds the user once", async () => {
+      const ds = newMockDataStore();
+      const userPool = new UserPoolServiceImpl(
+        mockClientsDataStore,
+        clock,
+        ds,
+        {
+          Id: "test",
+        }
+      );
+
+      const user = TDB.user();
+      const group = TDB.group({
+        members: [user.Username],
+      });
+
+      await userPool.addUserToGroup(TestContext, group, user);
+
+      expect(ds.set).not.toHaveBeenCalled();
+    });
+  });
+
+  describe("removeUserFromGroup", () => {
+    it("updates the group's members", async () => {
+      const ds = newMockDataStore();
+      const userPool = new UserPoolServiceImpl(
+        mockClientsDataStore,
+        clock,
+        ds,
+        {
+          Id: "test",
+        }
+      );
+
+      const user = TDB.user();
+      const group = TDB.group({
+        members: [user.Username],
+      });
+
+      await userPool.removeUserFromGroup(TestContext, group, user);
+
+      expect(ds.set).toHaveBeenCalledWith(
+        TestContext,
+        ["Groups", group.GroupName],
+        {
+          ...group,
+          LastModifiedDate: clock.get(),
+          members: [],
+        }
+      );
+    });
+  });
 });
diff --git a/src/services/userPoolService.ts b/src/services/userPoolService.ts
index fbe0a885..f83e73c0 100644
--- a/src/services/userPoolService.ts
+++ b/src/services/userPoolService.ts
@@ -138,6 +138,7 @@ export type UserPool = UserPoolType & {
 export interface UserPoolService {
   readonly options: UserPool;
 
+  addUserToGroup(ctx: Context, group: Group, user: User): Promise<void>;
   saveAppClient(ctx: Context, appClient: AppClient): Promise<void>;
   deleteAppClient(ctx: Context, appClient: AppClient): Promise<void>;
   deleteGroup(ctx: Context, group: Group): Promise<void>;
@@ -346,6 +347,27 @@ export class UserPoolServiceImpl implements UserPoolService {
     return Object.values(groups);
   }
 
+  public async addUserToGroup(
+    ctx: Context,
+    group: Group,
+    user: User
+  ): Promise<void> {
+    ctx.logger.debug(
+      { username: user.Username, groupName: group.GroupName },
+      "UserPoolServiceImpl.addUserToFromGroup"
+    );
+
+    const groupMembers = new Set(group.members ?? []);
+    if (!groupMembers.has(user.Username)) {
+      groupMembers.add(user.Username);
+      await this.saveGroup(ctx, {
+        ...group,
+        LastModifiedDate: this.clock.get(),
+        members: Array.from(groupMembers),
+      });
+    }
+  }
+
   public async removeUserFromGroup(
     ctx: Context,
     group: Group,
diff --git a/src/targets/adminAddUserToGroup.test.ts b/src/targets/adminAddUserToGroup.test.ts
index 1061e8c7..c323d171 100644
--- a/src/targets/adminAddUserToGroup.test.ts
+++ b/src/targets/adminAddUserToGroup.test.ts
@@ -22,7 +22,6 @@ describe("AdminAddUserToGroup target", () => {
     clock = new ClockFake(originalDate);
 
     adminAddUserToGroup = AdminAddUserToGroup({
-      clock,
       cognito: newMockCognitoService(mockUserPoolService),
     });
   });
@@ -43,41 +42,11 @@ describe("AdminAddUserToGroup target", () => {
       UserPoolId: "test",
     });
 
-    expect(mockUserPoolService.saveGroup).toHaveBeenCalledWith(TestContext, {
-      ...existingGroup,
-      LastModifiedDate: newDate,
-      members: [existingUser.Username],
-    });
-  });
-
-  it("adds the user to a group only once", async () => {
-    const existingGroup = TDB.group();
-    const existingUser = TDB.user();
-
-    mockUserPoolService.getGroupByGroupName.mockResolvedValue(existingGroup);
-    mockUserPoolService.getUserByUsername.mockResolvedValue(existingUser);
-
-    const newDate = new Date();
-    clock.advanceTo(newDate);
-
-    await adminAddUserToGroup(TestContext, {
-      GroupName: existingGroup.GroupName,
-      Username: existingUser.Username,
-      UserPoolId: "test",
-    });
-
-    // try add a second time
-    await adminAddUserToGroup(TestContext, {
-      GroupName: existingGroup.GroupName,
-      Username: existingUser.Username,
-      UserPoolId: "test",
-    });
-
-    expect(mockUserPoolService.saveGroup).toHaveBeenCalledWith(TestContext, {
-      ...existingGroup,
-      LastModifiedDate: newDate,
-      members: [existingUser.Username],
-    });
+    expect(mockUserPoolService.addUserToGroup).toHaveBeenCalledWith(
+      TestContext,
+      existingGroup,
+      existingUser
+    );
   });
 
   it("throws if the group doesn't exist", async () => {
diff --git a/src/targets/adminAddUserToGroup.ts b/src/targets/adminAddUserToGroup.ts
index 5d8ebbcd..19e8a19d 100644
--- a/src/targets/adminAddUserToGroup.ts
+++ b/src/targets/adminAddUserToGroup.ts
@@ -1,18 +1,14 @@
 import { AdminAddUserToGroupRequest } from "aws-sdk/clients/cognitoidentityserviceprovider";
 import { GroupNotFoundError, UserNotFoundError } from "../errors";
 import { Services } from "../services";
-import { Group } from "../services/userPoolService";
 import { Target } from "./Target";
 
 export type AdminAddUserToGroupTarget = Target<AdminAddUserToGroupRequest, {}>;
 
-type AdminAddUserToGroupServices = Pick<Services, "clock" | "cognito">;
+type AdminAddUserToGroupServices = Pick<Services, "cognito">;
 
 export const AdminAddUserToGroup =
-  ({
-    clock,
-    cognito,
-  }: AdminAddUserToGroupServices): AdminAddUserToGroupTarget =>
+  ({ cognito }: AdminAddUserToGroupServices): AdminAddUserToGroupTarget =>
   async (ctx, req) => {
     const userPool = await cognito.getUserPool(ctx, req.UserPoolId);
 
@@ -26,16 +22,7 @@ export const AdminAddUserToGroup =
       throw new UserNotFoundError();
     }
 
-    const groupUsers = new Set(group.members ?? []);
-    groupUsers.add(user.Username);
-
-    const updatedGroup: Group = {
-      ...group,
-      LastModifiedDate: clock.get(),
-      members: Array.from(groupUsers.values()),
-    };
-
-    await userPool.saveGroup(ctx, updatedGroup);
+    await userPool.addUserToGroup(ctx, group, user);
 
     return {};
   };