Excellent hack! Two feature suggestions which should be painless to implement.
First, the
Not logged in/Unregistered usergroup needs to have the View Forum permission set to Yes; otherwise, invitees get a permission error when they try to register; can this be fixed?
Second, users who are invited and register are automatically thrown into the
Users Awaiting Moderation usergroup. It would be nice if you could set which usergroupid a newly registered invitee is thrown into.
Also, I think it would be more conveniant, at least for me

, if instead of setting which usergroupids ARE NOT allowed to invite people, being able to set which usergroupids ARE allowed to invite people.