I haven't seen anyone in "costumes" for the Christmas party, but like Beattyfamily said, most are dressed festively. My husband and I have Mickey & Minnie Santa hats that we wear, but other than that, we just dress warmly.
Our family bought sweaters to all match for the party...then decided we would go for halloween instead this year...but in previous years It's casual festive...hats' t's...no costumes...
Like the others on this thread. I usually wear one of my Disney Crhistmas sweatshirts, or buy a new one, along with a Santa Mickey hat. I have seen other families wearing matching sweatshirts before getting their family portrait taken...or at least matching colors.