Great way to destroy your house, lose a good customer, and lots of other problems.
Space isn't the issue. 20 people you don't know is. You have no idea how they'll behave or what they are capable of.
Tell them you are too busy during the holidays with your own families and friends to take on more.
There's likely a good reason they haven't rented a back room at a restaurant or similar venue.
LTG : )™