A beef or lamb casserole, or even a chicken one - with a tomato based sauce.
Something like smoked fish and broccoli in a white sauce.
Both made in advance, and reheated in oven in lasagna dishes (so serve straight to table/buffet spot for people to help themselves).
A large bowl of mashed potato (can be done as above - or spuds peeled and chopped small in advance and just boiled/mashed quickly at the time).
Large bowl of green salad, and a separate bowl of cherry tomatoes.
Maybe some crusty bread or garlic bread alongside.
Roasted chicken joints (part boned breasts or thighs are both good for this), done on a bed of veggies (onion, pepper, tinned tomatoes etc) with roasted baby potatoes. Can prep in advance and good enough to serve to table for people to help themselves. Or to serve up to them formally. But doesn't feel so formal as a roast and not a lot of running around when the guests are there.
Do they eat rice?
A chicken and mushrooms in white wine and cream sauce (can actually serve with rice, pasta or potatoes).
A curry for those who will tolerate spice.
Or a Chinese sauce for those who won't.
A beef stroganoff.
Again - pick 2 and let people help themselves.