Here is a paddleboard that can accommodate 4 riders. It has a 8″-thick PVC construction and 17′ L x 5′ W surface. This paddleboard can support up to 1565lbs of weight. The top of it has a large EVA foam foot pad to provide you a slip-resistant surface.
You get elastic bungee cords at the back to secure your personal belongings. The paddleboard has 4 fins for straight steering. Not cheap though.