[gringochileno]

While it's likely that a girl eye-fucking you while grinding on another dude is just an attention whore, if you want to hit on her anyway you really just need to wait until the guy gets distracted or leaves her alone for a minute, then hit her with your opener. She'll still probably reject you to feed her ego because that's what little cockteases do, but your chances are better than if you try to talk to her while she's actively flirting with another guy.

Here's a good breakdown: http://www.rooshv.com/how-to-pull-a-robbery