Wedding ballroom dancing scene near end

I think it was supposed to be Volmer's court, who have been supporting him for hundreds of years in exchange of staying young like him. Some were part of the sanitorium staff, the others probably came from around the castle.

They were not patients.

I was confused with this part as well. Then I noticed a few familiar faces (female nurses and head Dr) and came to the conclusion that it was his staff. Maybe his staff was in on the experiments so they could stay young?