After you opened/started a game together once, you cannot (or at least we could not) send an invite over steam.
The problem was i had to set my status to "visible for friends" in the settings in game... which took us 40min to figure out.
Very bad design (you might call me dumb, maybe I was - but only after failing each possible attempt to make my friend join for 40min i thought "it could not possible be an ingame setting"
thats how trusting I am into newly developed stuff...)
