Well, first off, you don't have to be American (native or otherwise) to speak English
For example, I'm Australian, and I consider English to come from England. Pet peeve of mine. Anyways...While it's good form to not go straight for the obvious "target" and keep people wondering and interested about you, sounds like you're clearly just getting comfortable talking to the guys and spending way too long doing this. Eventually you're going to have to step out of your comfort zone and direct the conversation to her some way.
Ah see I knew you were doing this out of fear. Well if she's with her boyfriend, this will be made known to you very quickly. If all you're doing is talking and people can see that, this shouldn't be a problem, especially in a group. The boyfriend will put his arm around her, or she will mention "Oh have you met my boyfriend, his name's Jack" and then you will know. If you bail too quickly at that point, it'll be awkward, but you should just let the conversation run it's course and then politely move on and say you better be getting back to your friends, nice chatting, have a good night.
You may still be in the polite talking mode, where she's talking to you, to be nice, because you seem nice, polite, haven't done anything wrong - and yet are not standing out in any way. You should challenge yourself to think of interesting topics, try leading questions such as "what would your dream job be" but it shouldn't be out of the blue. Basically you should use the conversation to show how passionate you are about topics (assuming you hit on one that you are both passionate about) and setting up another meeting, preferably regarding one of the things you're going to do together that you were talking about.