Do they have a private dining room for 20 people or so

Not really private so much as an area that could be assigned a group of 20.

Im not sure if they have a completely separate room but you could probably use the hack right area but they don't take reservations

I am not sure.... there is the main restaurant that is always full but there is a smaller more intimate area away from the main area that may work for private dining. I would call and ask