Group messaging applications are all the rage these days. Everybody has one or wants one. GroupMe currently allows non supported ( like Windows Phone, currently) platform users to use SMS instead to communicate with their way. An application would be better though, GroupMe. GroupMe utilizes the contacts API so you'll have to wait till the mango update. We'll keep you updated on the status of this much awaited app.
Update: Groupme for WP7 is now available in the marketplace. Download link below: