First of all ow many bedrooms / sofa beds / floorspace do you have?
I'd consider putting teens in a tent in the garden to make space for the adults. Hot water bottles and sleeping bags can make a tent cosy.
Are you intending to do the whole thing or are the guests going to be involved?
Empty your freezer now and then start prepping veg and freezing, so potatoes already in a tin that can go straight in the oven. The same with parsnips, carrot and swede - whatever your normal veg is.
Cook two meats, one in the oven and put the other in a slow cooker.
Christmas eve (or arrival day) - do a 'Jacob's join' ask each family / couple to bring a dish to eat. Order a couple of pizzas.
Breakfasts, I would do a buffet, get those multipacks of cereal, chop up some fruit, put out a toaster and a couple of loaves of bread, some ham and cheese.. You can put the cereal and bread out the night before and then add milk and fruit in the morning.
I'd do this every morning maybe changing things a bit like different fruit.
Have a 'snack box' somewhere filled with crisps, sweets, bags of nuts, boxes of raisins - teenagers have hollow legs.
I'd consider hiring some professional equipment like a catering hot water tank.