You can add an additional driver at the exit booth. Correct there is no charge for spouses as long as they have a license and credit card in their name and have the same address (don't laugh, we get the "this is my spouse" all the time with different addresses, so, I thought I would toss this info in.)
Another FYI, if you join Insiders, additional drivers are free. Alamo and National are owned by Enterprise, we share the cars. So, I would go with what is the least expensive.
What is Insiders and how do you join? How many additional drivers can you add? TIA