Systemlink has the following quote from the Official Playstation magazine where it mentions some of the new multi player options in GT5. Its sounds like Polyphony are really trying to build a solid foundation for Gran Turismo online to encourage more of a community.
"My Lounge is GT5's hub from which you'll access all of GT5's new online features. Hit the Community tab and you'll pull up a bulletin board filled with your latest stats and your recent mail from friends and other players.
You can browse through your friends' bulletin boards and check their progress on and offline, and sent out mail and challenges. My Lounge supports voice chat for up to 32 players and lets you host 16-player online races or spectate on races in progress, as well as watch the GT-TV VOD service and race replays."