There are many reasons why a girl might have a BF. She's not always really interested in him. E.g. she can think it's not cool to be alone so it's better to have a poor BF then noone.
And if she asked YOU out then she definitely is interested. (Well, almost definitely - you never know with a woman
).
I see you're in doubt... I'd say you need to learn to be a man i.e. to make decisions. If what you decide doesn't work well then at least you've learnt something. Don't put that much pressure to the whole situation. Just make what you feel is best and have fun. This way the girls will feel better around you as well.
Act as if she were single. I'm just meeting a girl who's got a BF. At the beginning she was talking a lot about her BF and now she only
says how she's stuck with him and how boring he is. LOL (I don't want her to be my GF though because I'm here for summer only and I live on the other side of the Earth).