I've been groped several times in clubs/bars/at parties by both males and females. It was never a fun experience because I was never out looking for something like that to happen. When it did, I always wondered how someone could be so brazen as I would never think of touching someone, let alone grabbing their butt, chest or crotch, without permission. Apparently though there are people who do this. Anyway, as for what I did, the one time it happened with a guy I was in a somewhat crowded hallway at a club. A dude shoved me against the wall and attempted to kiss my neck. I got my hand between us and shoved him back about 2 feet and yelled in his ear, "not interested". The other time I was djing in front of about 3000 people and some chick had busted a reach around on me. I turned around and there were actually 3 chicks behind me and I had no idea which one did it. Since there was no way to even be heard over the stacks of speakers in the place I just shook my head and mouthed "don't f***ing touch me". They buggered off luckily. The third time it happened I was at a large party in the Hollywood Hills. I was sitting on a couch and fairly inebriated when this chick sat down next to me and started making small talk. I could tell she'd had quite a few as well but I was not giving any indication at all that I was even remotely interested. I wasn't even really responding just staring off into space and she leaned over and grabbed my crotch. I just grabbed her hand, gave it back to her, and got up.
I guess any time something like this happens you kind of have to size up the situation and decide how you want to act about it. I would think that unless you are at a specific pick-up bar or some place that's known for "cruising", etc. that kind of unwanted advance would be just that, unwanted. But since it's happened to me multiple times I'm really not so sure wtf some people are thinking. You probably did the best thing you could do, which was just ignore it. If you're there and it happens again see if you could see who's responsible and tell them "no thanks".