I don't think that directly aproaching her in such situation would be good. Anything you would say wouldn't separate you from the others who had aproched before you. She would be annoyed with your presence, the moment you would be in front of her...
What I would do is this... I would place myself somewhere close to her (a few meters maybe) so as she would be able to see me. I would be with a friend (or not?). Then I would look at her but without a smile, just an ordinary look. Here what I do is, as you can not stare at her all the time untill she looks in your direction (that would be totally weird/frightening), I would look at her from time to time (often), shorly, and eventually our eyes would meet. Than from her reaction I would know if there is any chance (maybe she's not in the mood tonight whatever)... If she looks at me for couple of seconds, purely out of curiosity (why am I looking at her etc.) that MIGHT be a good sign and I would see her future reactions (she might look at me again !!
). If she looks at me later that's GOOD. You could give her some sign with your head/eyes/smile during your later or even first eye contact, depends on your and her type of personality (means do what you feel would be appropriate). If she's totally uninterested (turns her head away as if nothing happened), then I'd forget her.
This I do often (or maybe allways), try to get her attention and then evaluate her reaction, before I make any move...
P.S.
Your facial expression is up to you, but I know my serious facial expression would separate me from the others...
Edit:
If you still learn then just aproach and learn...