You can add both during the online booking process. You will find them under the EXTRAS section once you have chosen your car model. Sometimes, on the search results page on the left-hand side menu, additional drivers may appear under the ''Deals and Features''. If they do appear tick the box to filter out all other options. They are often free of charge.
If you have already made your reservation you can login to: -
viewmyreservation.com
Using the email adress you made the reservation with and the booking reference number. On the left-hand side menu choose AMEND.