It's not a demo at the moment--it's a multiplayer-only beta. So yes, it has full multiplayer support.
It's also very stable for a beta, which is a nice bonus.
If you own Soulstorm, you can access the beta now. If not, you'll need to wait a few days.
The beta is available right up until the game is released.
If you have Soulstorm, check out this link for how to access the beta today:
http://community.dawnofwar2.com/view...hp?f=43&t=3649
If not, most of those steps still apply, but you'll have to access the beta as you would normally through Steam.