I think it would be nice if creators of social groups could send requests to join to the groups to particular members. Particularly so for the GTPlanet Fan Club which surprisingly doesn't have the most members yet!
Yes, I'm afraid a built-in invitation system invites abuse, and even legitimate invitations would probably not be looked highly upon by a lot of users. Of course, there is nothing stopping you from inviting your friends to join a group via profile post or private message.
