I would invite her, but it doesn't mean your DD has to pal around with her all day. We had DS' 7th birthday at a trampoline place. He greeted everyone has they arrived and said goodbye to everyone as they left, but during the actual party, I noticed he stuck pretty closely to a small group. And other kids grouped off. It wasn't 12 kids moving around in lockstep. Sally can go gossip with whoever she was gossiping with in the car. Everybody's happy.
ETA: It sound likes this is a pretty big party. Nine girls from this one class and then other girls from the other three 3rd-grade classes? That's big enough that inviting Sally isn't really a big deal.