Oh god, no do not do what Diablo suggested I have been in this situation before except there were three of us who liked the same girl. There is no point in asking her out if she knows she has a choice between 2 guys. I ended up going out with the girl in the end because I wasn't really trying to go out with her unlike my other 2 friends. They both asked her out, and she said no to both of them because she wanted to go out with me since I was a challenge.

Keep yourself at a reasonable distance, be funny around her and show her your best when she strikes up conversation with you. This will make her start to chase you assuming your best is good enough, drop a few subtle hints to let her know you may be interested in a relationship. Though make sure when you are around her that your personality dominates your friends to a certain extent, and make sure you direct the course of conversation.