Can't play with my friend.

[Abrinak 0]

I bought ARMA II Combined Operations sometime in June and I wanted my friend to get it too, he wasn't sure whether he wanted it or not so I went to his house, logged onto my steam account and installed it on his computer so I could show him the game and let him try it out for a bit, now that he has bought the game whenever we try to join the same server one of us gets kicked is there any way to fix this?

[Scene 36]

Sounds as if his copy of the game is still using your CD key from when you installed it on his computer.

Not sure but try checking the CD Key for each of your games in steam by right clicking on the game in your library and selecting "View game CD key".

[Abrinak 0]

He has different ones than I do but it still wont let us play together.

[Diamondwing 1]

Im currently having the same issue. we were able to play together before 1.7.2 and now that the patch hit it won't let someone connect that's on the same router unfortunately. Trying to fix this but it's rather difficult when you know nothing about routers.