Not sure if this should go in the multiplayer forum or what... That seems to be all about people looking for others to play with. Anyway, here goes.
How does sharing a world work? I have a friend on my buddy list but he hasn't sent me an invitation. If his game is "online", can I join him? And do you have to send invitations every single session when you play a world? If not, is there any way to have a standing invitation?