Create a VPN between you and your friends and you should be able to play online(!) multiplayer together. My friend and I were using openVPN but others like Hamachi or Tunngle should work, too.
This method kind of sucks if you just want to play a random online game but it should be good enough for playing the campaign together with some friends.

Just putting this here for visibilty since many people were looking for a way to play multiplayer.
Feel free to comment, if this works for you or leave other suggestions.