Tris Hussey has written an interesting article “How To: Build a private collaboration site on WordPress.com in 5 minutes” on how to build a private, WordPress-based collaboration website.
There is also info for self-hosted WordPress users, using the P2 theme.
Full Story: http://thenextweb.com/lifehacks/2010/11/26/how-to-build-a-private-collaboration-site-on-wordpress-com-in-5-minutes/