Wow a lot of ridiculous shit is being said in this thread. This is being way too overanalyzed.

The bottom line is - she's showing interest in you, either because she really is interested or she just wants attention/fun. She's stopping there because she has a boyfriend, and she doesn't want any blame/guilt on herself. Girls like to rationalize things in this way. She'll invite you but nothing more. The ball is in your court buddy.

So you can just fuck it and go after it, or accept that she has a boyfriend and chill. The morals of the situation are up to you to evaluate. I personally don't believe in going after girls that have boyfriends but sometimes shit happens.