Dude...
Been there.
Just go ahead, talk to her ALONE, just both of you, and say to her "Look, there's something important I need to tell you: I like you" Straight to the point, add whatever you want; just don't act like a pussy, remember that YOU are the prize, and that's the attitude you need to have, there are 3 possibilities: 1) She likes you too: Fuck yeah! be happy
2 and 3) She'll think about it OR rejection: Be ready for that; if you wanna keep the friendship, don't overreact, just say "Ok" and be normal whenever you are with her, just like before, and remember to move on...