It is often free, but not always. If you make a search of this website and on the results page ( where all the available vehicle photos are displayed) you will see a small icon near the car photo, if a free additional driver is offered. Also down the left-hand side menu on the same page you will see " Deals and Features" , it will also be mentioned here. If there are no offers , then you can add an additional driver under the EXTRAS section, where you find also Baby Seats, GPS, Ski racks etc.. You can reserve by ticking the box and pay on arrival at the desk.