An additional driver is anyone different from the person who pays for the rent (main driver), who has the right to drive the vehicle.
Additional drivers must be registered. Otherwise, in case of an accident, if a non-authorized person was driving the rented vehicle (except in case of robbery), the insurance coverage would be void, and the main driver would be liable for every damage.
To register an additional driver, he/she must be present when the rent contract is filled. Also, he/she must comply with the same requirements as the main driver.