Usability Yes. Social Features, Uh-Oh.
Fragment of a discussion from Talk:Question of the week/Archives/2009-12-14
@Randomran
I don't see why anyone could not join any groups as long he/she is an autoconfirmed user. There should be groups log showing who joined or left and when.
The less strict for someone to join a group and broader range of opinions it will permit within this group. So i'm against the idea of "invite only".