1. you could create your one specific intro page with login/pass/reg only form and make this page as home in options of wp mu. i think.
2. there is flickr plugin live allready (hope gallery will be added lately to host photos inside BP too)
3. you can use phpbb or other forums insteed. i think it’s even better )
4. i think there is invite system live in BP, but i didn’t use it by now )