A small number of largely uninteresting things are a little better with more people.
Specifically, the game has a cart (read: wheelbarrow) that is a bit rough to use without a road, and one way to make a 'road' is to have 2 buddies running ahead of you smoothing as you haul it. Not quite as intense, using boats is better with friends too, so one guy can steer and another shoot if you get into a fight.
neither is critical. the cart is of minimal use since most areas the terrain is simply prohibitive of using it at all, and boating is perfectly doable by letting go of steering for a moment to shoot, after pointing in a safe direction (which could be just to crash it into the shore, but that is a tactical decision for another discussion!)
Other than vehicles, bosses are easier with friends, but they are easy enough solo if you prepped, which is what the game is about anyway.