Thanks to lolwaut, I found the trick to getting the Friendship trophy. I only had 3 PS3 controllers, but wondered what I could do for a 4th. Here is what I did. I turned on three regular controllers along with my RB guitar. I just used the RB guitar to select a vehicle, then didn't do anything the rest of the game (only controlled one vehicle). I ended up getting the trophy too although only one finished.
So if people have accessories, use them.