Unfortunately, having the same issue. Haven't been able to play a coop game yet.
Have tried coop by:
- clicking "join" from the menu
- accepting a steam invite while in game
- accepting a steam invite while in the main menu
- accepting a steam invite while the game isn't loaded
- joining friend's game via Steam's friend list while in game
- joining friend's game via Steam's friend list while in the main menu
- joining friend's game via Steam's friend list while the game isn't loaded
and vice versa.
No luck so far.
