i might be blind, but i cant seem to find where members can join a group that has been created? do you have to add them manually or can the request to join?
When creating a new usergroup you have an option to make the group publiclly joinable. Users can then join the user group through UserCP->Group Memberships.
You can also set the usergroup to moderate join requests. ie they won't be in the usergroup till you (or group leader) says so.