I don't know if you're going to get a response from the perspective of someone who was groped, but from the perspective of someone who has groped, I can tell you that the only trick is knowing which girls will put up with it.
It's not something most women will let happen. They'll put up a fuss and make a lot of noise. But the kind who will take it are easy to spot. There's just something in their body language and demeanor that screams victim.
The thing is, unlike you, most girls who will put up with it, will try to push hands away (however ineffectively) at first. The one's who are like you who don't even do that, well, it's almost a guys duty to see how much he can get from you.