Getting this issue sometimes too, especially when having a bad connection day and getting signed off PSN than signing back in, usually a game restart is needed to fix it.
Try to clear you game cache and restarting the system. Also, turning off the network connection via your PS3 and then turning it back on again worked for me.
I think the network has been having problems. Lately there has been more lag, black screens, crashes and people not see each other on the track. Last night I quit playing because the lag was so bad.
This has to have something with a bad connection or happens just after you sign off(or forcibly gets signed out) your PSN after you entered the online lobby to start creating a new room not perceiving anyone other's juncture for the muddle of your own.
This problem can be solved easily by reseting the system and getting the power of your connection live somehow(or sign in the PSN back after the trouble happens), I've received the same problem a couple of times for the same reason after I came into the online lobby with my connection being fine.
