You can add an additional driver in the second step of the booking process when making the booking online. If you already have a booking and need to add an additional driver, don't worry. Just inform the agent at the counter when you are picking up the car, and they will be able to sort this out for you.
The person who is being added as an additional driver will always need to be present at the rental counter with their full valid licence(s) in order to be added to the rental agreement.
If additional drivers are added at the rental counter an extra cost will be added at the pick-up time. The exception is if you have a free additional driver included in the cost of your booking. The additional driver will still need to be present and added at the counter, but you will not need to pay for it.