groups help in maybe 2 places, a little bit:
1) boating, to have a driver and a gunner, if you get attacked. Not necessary usually, but it helps.
2) carts. The cart is clunky and having some help to push it around or clear a path is nice. Also not necessary.

I don't recall if it scales bosses by groups or if that speeds up the giant health pool slog some. Meh. They can be done solo.

Ive mostly played solo, many many hours of it, its great.