Not as hard as it sounds
First of all I am mainly providing a location and yes a good portion of the food but everyone contributes a dish or something. These are usually pre-prepared and ready to eat when they arrive
I do some of the cooking (25 lb turkey and the dressing) but others will cook the sides. The hard part is making it all come out at the same time. It never does but it is close enough to work
Frankly cooking one dish for 20-50 is probably harder than a Christmas dinner. It takes a huge pot, a massive amount of chopping and dicing, etc etc and a long cook time