"[Messages page] works more like a message board than like e-mail. The main difference between Multiply and other message boards is that Multiply includes all the discussions going in throughout your network, as opposed to being focused on one specific topic. At any time, you can "adjust" the messages page to show you less, or more, of what's going on in your network."
Looks like you could read all of your groups and friends via this. Subscribe to a thread via email is a really nice option. Now, if they only had rss for these!