Haha, the thread still lives! I remember this was one of the first threads I read on this Social sub forum.
Oddly enough I found myself in sort of similar situation this last weekend. I was at some crappy club/lounge called Katra for my girlfriend friend's birthday party. Decent mix of people although quite crowded, place was dark, drinks were expensive, and the DJ really sucked.
Girlfriend was dressed VERY nicely and had dolled herself up in makeup which she rarely does. She went to the restroom while I was at the other end of the place, near the table talking to some new acquaintances. Apparently on her way there a black guy (she didn't get a great look) grabbed her ass. Not just brushing it or touching it, but full on grabbed her butt. When she came back from the ladies room she was visibly upset and told me about it. I asked her if she knew who it was or could point him out to me which she said she could not. I was pretty angry about it but not enough to walk up to random guys asking if they had grabbed my girlfriend's behind.
I'm a pretty tall and strong guy but to be honest, even if she did pick out the asshole, I probably would have just made some verbally scathing comments at the most. No sense in getting the place all riled up, embarrassing her friend/birthday party group, getting into a fist fight, and possibly shot/stabbed/hit with a chair etc. The smart thing would probably be to talk to management about it; sexual assault, police, etc and ask them to remove the offending party.
TL;DR No need for fisticuffs.