How would you react?

Until there is a ring [or relationship is close enough to having rings involved], everyone is fair game. That just about sums it up for me :)

I'm currently in a long distance relationship. and if a guy were to pursue me, I'd just plain out and tell them, "I love my boyfriend so much! So, it'd be a good idea to back off."

But since you've hung out with this girl a couple of times and she has said nothing about the relationship, then she might not be taking that relationship all that seriously. However, I don't really know from the information you've given. If it's just been a casual hanging out and there were no signals she was sending. then she just wants to be friends. And you should respect her decision. But, I wouldn't pursue her romantically anymore. You'll just end up incredibly frustrated.

I would probably keep in touch with them, as a friend, if they seem interested in me.
But I wouldn't continue to pursue them romantically since they're in a relationship.