have no fear ptb, AT&T did very little messing around in WP7. In fact, you can uninstall any trace of AT&T on your phone, but thats besides the point.
Go to the market and search "xbox live extras"
Go to xbox live
and right next to the picture of your avatar where it shows your gamer tag above your gamerscore, click on your gamertag and it should send you to more xbox options, including your friends list where you can view/send messages to your online and offline friends.