Hi D.,
I think O. of the hardest parts of being a parent is watching our kids behave in a way that we, for O. reason or the other, think is different than what we have pictured in our mind for them. All kids are different. My son tends to be on the shy side, too. He's only 5, but his first impulse in a many-kid situation is "nah....". :) All kids are different. Sometimes what we perceive as shy can just be an intelligent observation period. I would leave her alone. Make sure she has opportunities to meet different kids (sports, activities, clubs, etc.) but don't push her. In a situation as you described, maybe bring something for her to do quietly (a book, craft project, etc.) by herself in the event that she doesn't want to join in. Maybe tell her when you leave the house that we are going to ABC for XYZ amount of time and in case she doesn't want to play with the other kids, she should pick something to do to pack up and take with her. Just an idea.