1. Click Add under Admin, Roles and Permissions then select Supervisor as the Base Role.
2. Add the "Create User" permission then click save. You can also enable the "Inactivate User" permission as well if you like.
3. Assign the Group Admin Role to a User that supervises a Group.
4. Login as that User to see that they can only add User's to the Perth Group that they Supervise. They can also add Users to subgroups as well.