"Can I fuck your bestfriend, I mean it won't mean anything, I just crave other dick even though we're in a relationship".
That's really what played in my head when I was reading this. You should slap that bitch around a couple times. She's probably already cheated on you, if she's asking if she can grind on guys and she hangs out with guys all the time, their is a problem.
Break up with that, and find someone that's actually wifey material. Instead of a bitch like that.
Dude, you need to put some stadium-sized testicles on. Confront her with what you think, and make that bitch know how you feel. If she does not give a fuck, then you go and screw one of her friends before breaking up with her. Of course, the friend has to be a slutty one- but at least one of them has to be a whore. After you fuck her you take pictures of the moment and post them on your soon-to-be-ex-girlfriend's wall. Man up, show her who has the dick in the relationship. And by the way you wrote about the problem, I think you deserve/will find someone better.
If my girlfriend cheated on me I would dump her instantly. I don't personally mind her talking/hanging out with other guys, but grinding and that type of stuff is way too far. You can find better. If she feels comfortable doing that even though you 2 are dating, she isn't worth the trouble. Any girl who can not have any remorse from doing that is just a bad girlfriend in general.