I think get "/userGroups" should only return groups. get "/userGroups/1/users" or "/users?group=xx" will show all users in one group.
I feel adding subsets in restful API is not good because the level can high.
```
A-B-C
   -C
   -C
 -B-C
   -C
   -C
 -B-C
```
In this case, if we return A, we may need to send a lot of data.