When I try to play multiplayer in the ME3 demo, I get this message:
You do not yet have access to the online features of the Mass Effect 3 demo. Please try again later.
Back on the main screen there is this scrolling text:
Activate Alliance Network to Access Online Features
How do I do that? All I can find is speculation that it requires DLC or that you need to wait until the game is released, but obviously it's already been released.