Uber does not have any policy regarding this. However, it's your vehicle and if you want all passengers to use a seat belt then you can enforce it.
Also consider state laws. If it requires everyone in the car use a belt, don't start driving until everyone has buckled up.
Actually, Uber doesn't have law or rule regarding that but I think it's safe if every passenger in your vehicle use the seat bet. After all that's what seat bets are made for, so it's not compulsory but I'd say it is necessary.
Uber’s Seat Belt Policy: Uber urges all passengers to buckle up, whether in the front or back seat of a ride.
While most people nowadays make sure children in the rear are in car seats or buckled up, it’s fairly common for adults in the back not to wear seat belts, especially when they’re in taxis and ride-hailing vehicles such as Uber and Lyft, transportation safety officials say. The consequences can be deadly.
In Portland, Ore., for example, a ride-hailing passenger was killed in April when a pickup truck crossed a median and hit the Lexus SUV in which he was riding in the back seat. He was not wearing a seat belt and was ejected through the windshield.
It isn't an enforced rule but you should still require it yourself for the safety of yourself, your car, and the people you are driving. If the car flips or anything like that, you can get very injured from them flopping around in your car.