That poor girl... honestly.. If she isn't okay with you touching her butt, then don't. The reason why she is "cool" later is because she isn't gonna have a bitch fit just for that. And it sounds like she might like your friend.

Being a girl, i can kind of understand her. Oh, and just because she "lets guys touch her", it doesn't necessarily mean that she likes it.

You guys actually kind of sound like jerks. Stop ass groping and tit grabbing and give her some respect