It depends on what insurance policy you have, but usually named drivers cannot drive someone else's car (even with their permission). It's best to check with your insurance firm who will clarify the situation. In terms of driving a rented car, this is different as it has its own insurance. When you rent the car, the person hiring it is insured but anyone else wanting to drive it will have to be added to the insurance (usually at a slight extra cost) as an additional driver.