Taking passengers with you in your rental car is allowed and at no extra cost. When you rent that car, you rent the vehicle, not the seat. Make sure you inform yourself about the amount of seats your rented car has though. Passengers are required to sign the terms and conditions of the rental contract, and if your passenger is underage, their parent or legal guardian will have to either be there with them (to sign), or provide a declaration of consent. Reach out to us if you need a template.