Big deal! She simply asked you to join her for a drink, not to marry you or go to bed with you. It doesn't have to be romantic or anything; just two classmates enjoying a break from studying. I say join her; all it'll cost you is an hour of your time and a couple of bucks for the drink. It's very small investment on your part, so the loss is minimal, but the potential return is tremendous. Any girl showing sincere interest in an aspie guy has to be a nice, friendly person who's fun to be around. After all, underneath all that aspergian garb, we're all human beings for whom another person's company is a basic need.
You said you don't like her romantically. But for an aspie guy, a girl showing interest is a pretty rare thing. So give the girl a chance. At the very least, you'll enjoy a good conversation with a nice person. Maybe you'll get to like her after you talk to her. I've had that happen to me many times. When you two are sitting together, pay attention to how she'll act. Is she smiling a lot? Does she touch you (e.g. on the arm) at every possible opportunity? Does she make risque jokes? If you notice at least two of those things, she likes you; in that case, give her a chance! It could turn out to be a great relationship. If she doesn't do either of them, then she simply likes you as a friend. Then hang out with her, enjoy her company, and maybe ask her to fix you up with one of her friends.
Best of luck.