It's been a while but when you set up a multiplayer game you can add friends.
Interesting that you did not notice the game pop up in the Top 10 of most downloaded apps last year. Somehow I tried it back then but it could not convince me. For the better I guess when I see how desperately addicted you guys are now :p
Pitta, you mentioned that you mange your Gamecenter friends through the Lost Cities app. Bought it in the last sale as well but I am wondering how exactly you add new friends.
An iMessage pop up appears and you send the invite to the person you want to add.