My friend has this game on Steam and we wanna play together. I know that there's no limitations between Steam and GOG really.. but how easy is it to actually join each other's game in this? Can we search for a room? Or not? Also if anyone knows of any limitations between EU and US then I'd appreciate the answer.

Thanks :)

I played it with a friend on steam and we did the following: He opened a multiplayer-game and copied his session-id. Then he sended me the id via teamspeak and I joined the game.

It should work the other way, too. You open the game on gog and send him your session id.