My oldest was like this, he even went further and hated a lot of party entertainment, e.g. clowns, puppets especially, etc. Basically, he would know everyone but not participate - EVER. As he has gotten older he outgrew this but I did learn that this was just his personality, he doesn't like loud, he doesn't like people disguised, and he likes small groups. So really there is nothing wrong with him, it was just not his scene. Make sense? I wouldn't worry about it, just take her, hang out with her and if she participates - great, if not, then let it be. The bigger deal you make of it the more she will think there is something wrong with her and there is nothing wrong with her.
Good luck!