As long as an adult is driving and everyone is squarely seated on the cushions, the Fort popo don't seem to care. I've seen plenty of 4-seaters with more than 4 people.
If anyone is Pooh-sized it might be difficult to get 3 across on the seats. The popo won't allow hanging on while crouched down, sitting in laps (only infants), etc.
If you can get everyone's rear end firmly planted on the seat, you should be okay. The benches are the same front and back cushion types you see on most carts. I don't know the size of your kids and adults so you'll have to use your own judgement.