Re: restricted join
There is nothing built in yet to the buddypress suite of plugins. You would have to search for options on WPMU itself. There are a variety of ways. Off top of my head:
1) Have members email the site-admin about registration and then have the site-admin manually create their account (whole time with all registrations off)
2) Use a plugin like “Signup Question” and then make the question and the answer something that you only give out to members you want to join. Almost like “invitation” so to speak.
3) There are plugins that exist for the admin to “approve new blogs”, but don’t know of any that deal with user accounts themselves, but that might work.
4) Create a page that users can use to signup by giving all the required information to the site-admin and then manually create them (so they don’t have to email you).
In terms of WPMU, there really is a “hole” in the invitation or restricting registration. It would be nice if something was created for sure, but as far as I can tell, options really are limited on solutions already created.