something else for the cool tools thread.
I happened across this while playing with ning to make a dummy social network, such as you would do for conference participants. What Huddle does is allow you to create mini workspaces for project groups that are private to that group, but visible to the owner. Thus you could create a set of workspaces for individual project groups, which are invisible to other members network who are not part of the project team but visible to the network owner, eg a translation workshop working on different bits of the same text.
Their education flier can be downloaded from http://www.huddle.net/business-solutions/huddle-for-education, and there's a canned presentation at http://www.slideshare.net/ULCCEvents/fote-alastair-presentation.
The real value is being able to graft a more closed environment to an open environment, so that you get this mix of workspaces in a social environment. Certainly the idea of closed and open is extremely powerful and, unlike BuddyPress doesn't make your social network too social.
Certainly worth further investigation.