I like it when I'm good friends with a guy before any questions of dating even come up. So, like the other suggestions, I think it might be best to ask her to just hang out with you and a few friends for a while. Since she is still with someone, let her figure out what she wants. A lot of the time, girls who aren't getting what they want or deserve in a relationship will look for it somewhere else. Let her figure out on her own what she's doing and what she wants. In time, if she doscovers she does like you enough to break up with her boyfriend and start something with you, then she will. However, if you really want to be with her this bad, then just keep with the light flirting and being interested in getting to know her.
