Most ATVs are built for one operator. ATV seats are large, but that’s to allow the driver room to move when turning corners or maneuvering terrain. It’s not designed for additional people to ride. An additional person’s weight alters the ATV’s center of gravity. The center of gravity is the point where the machine is most stable. The lower and more centered the center of gravity the more stable the machine and less likely to tip over. When a person rides on the machine with the operator the center of gravity moves in the direction of the extra weight. This means the vehicle is more likely to tip over. This can also be true if you are hauling heavy items. Chemicals, hay, or feed being hauled alter the center of gravity similar to the weight of an extra rider.
When another person is riding on an ATV they distract the operator away from his/her main responsibility – driving safely. Operating an ATV takes lots of body movement. When there’s another person in close proximity, it’s more difficult for the operator to move appropriately.
Some ATVs are built for additional riders. If this is the case, the user’s manual will say so. The manufacturer took the extra weight into consideration when designing the ATV.