Ok, so I just got the game for the XBOX 360. My friend wants me to join him and he plays on a PS3. He seems positive that we should be able to play together, but I cannot find him or his clan anywhere. How do I play with him?

You cannot play online together across platforms, neither Xbox360 or PS3 allow this although some Xbox360 & PC titles can be played online together they are few and GRAW is not one of them.