If you want to add someone as an additional driver, they'll need to be with you when you go to pick up your car. The additional driver will also need to present their original driving licence in order to be added to the rental contract.
If you can't take them with you when you pick the car up, you can take them back to the counter and add them later on. The car hire company will charge you for the additional driver for the whole time that you have the car.
The rules about the main driver also apply to any additional drivers. So, if your additional driver is under 26 or over 69, please check your car's terms and conditions.