So it's... Yahoo groups? I hope you've done it a lot better than they did. Their interface is horrible. Almost every group I'm on, most people just use the email list because the Yahoo interface is so horrid.
It is similar to Yahoo groups but it can also be used like a secure reddit. So you can create a network, like Hacker News, invite who it is you want to join your network and then post (just like Hacker news) interesting articles that people in the network can then comment on. We also offer the users the ability to share files. So we hope we offer something different, and better, to Yahoo groups.
sounds like you could study Yammer for some inspiration.