I would guess,she is just being two. The "highly sesnitive" kids usually have more than just sound or over stimulation problems. And they don't have situations where they are okay, or comfortable, with a lot of activity or sound.
She may be overwhelmed by some situations, don't force her to participate, but don't leave because she wants to. Set a time when you will leave, empathize with her feelings, but explain that you want to have some time with your friends, or that it would be rude to leave so soon, and that she can just sit next to you until it is time to leave, she doesn't have to go participate, then don't really show her any extra attention while she is sitting there. I would guess that after about 10 -15 min. of being able to observe, she will be up playing. It may be hard to do at first with a "just" 2year old, but as she gets closer to 3 it will be easier.