Last night I created an online game and was playing split screen with my friend. I invited another friend on xbox live to join the game but he was not not able to join. I think I remember reading that you cant do split screen and online multi player at the same time. Is this true? And if so, will it eventually be possible to do split screen and online multi player?
Maybe you started an offline game. Try saving your map and the loading it again, then you can select Online game, that should do it. Splitscreen (up to 2 players per box) + online friend(s) should work then.