Whether there is an additonal charge for an extra driver depends upon who the extra driver is. Just double checked my copy of the Emerald Club rules that I received when I enrolled, and regarding authorized drivers it says that additonal authorized drivers include: (1) a member of your immediate family who permanently lives with you, and (2) a business partner, employer, or regular employee who drives the vehicle for business purposes.
If the additional driver falls into those categories, no need to stop at the counter to add an additional driver.
Also, to clarify the lawgs' comment about any car on the Emerald Aisle being available to you, I believe this is only true if you book an internediate or above (please correct me if I'm wrong about this). My understanding is that is you reserve and pay for a compact, you get a compact. However, if you reserve and pay for at least an intermediate, you can take any vehicle available on the Emerald Aisle, and you will still only pay the intermediate rate, even if you take something bigger.
One tip I found to be true is that full-size and intermediate often cost the same with National. Since you are only guaranteed to receive at least the category you reserved, you may as well reserve the full size rather than the intermediate.