Would the public usergroups in vBulletin work good enough for you?
Mods couldn't add members, but members could sign up and you could have your mods validate them, by adding them as group leaders.
Yes, that would be, is there anyway to make the usergroups public without recreating the groups? I ask because I don't really want to redo all my usergroups as it take me 1-2 hours recreating and setting the permission, etc.