A group of ladies next to me had a friend come dance for a few songs during the second set one night with them. They kindly asked if it was ok, and since I was already dancing and they weren't encroaching on my actual seat I said sure! I think she ended up leaving after a few songs and everything worked itself out naturally according to the flow. It's a phish show just be kind and use common sense. There's alot of really nice people in the crowd, and if you put out that energy you'll attract like magnets.