Question about a girl I like?

Ok so, I'm in a sort of half-band. I say that because it's like me, and a girl my age, and one a couple years younger, and we all play guitar and the girls sing. (I say it's a half band because there is no bass or drums or anything, and we aren't that great to be honest, except that they are pretty good vocalists, but that's not what this question is about lol) So the one my age has a boyfriend, or at least she did last time she had a school dance. I'm not really great friends with her, and I'm very shy and at some times awkward in what i say because the right words just never come out of my mouth >.< first, I really wanna be better friends with her, but i only really see her once a week if i'm lucky, and I don't have her phone number, so it's not like i can text/talk to her that often, especially not one-on-one, because whenever we practice, there's the other girl in the room, and our guitar teacher (because our teacher is really the one who put us in the half-band together), so i feel even more awkward then normal saying anything or asking for her phone number >.< any ideas? For now, while she has a bf, I want to at least get to be better friends with her, then maybe if they break up, i could turn into that other, back-up guy?