Depending on the state and/or codes utilized/Emerald Cub, there is no CHARGE for spouses as drivers- different from whether there needs to be a license on file.
I don't believe they need added from what I've read on the Flyertalk forum. ETA-I've read about 300 different thread about this with varying information. Sometimes they add the spouse at the gate, sometimes they don't and just hand the license back to you. I guess we'll all be there just to be sure.
Automatic Authorized Drivers
Free (in the United States and Canada) - An Emerald Club member's spouse, immediate family member, business partner, employer, or fellow associate who drives the car for business purposes.
Immediate family is a Mother/Father, Son/Daughter, or Brother/Sister who has the same address on the drivers license as the Emerald Club member's. This includes a spouse regardless of whether he/she has the same last name, is a common law spouse or same gender domestic partner.
Who May Drive the Vehicle - Authorized Driver
a) You represent that you are a validly licensed driver for the type of Vehicle being rented, at least 21 years of age (an Underage Primary Driver Fee will apply for drivers under age 25). National may verify that your license is valid issued and in good standing.
b) Unless applicable law requires otherwise, the Vehicle may only be driven by you or an Additional Authorized Driver. An "Additional Authorized Driver" is an individual who (i) is a capable and validly licensed driver for the type of Vehicle being rented, (ii) is at least 21 years of age (a young renter fee may apply for drivers under age 25), (iii) has your prior permission to drive the Vehicle, and (iv) is either a member of your immediate family who permanently resides with you, or a fellow employee who drives the Vehicle for business purposes.You will not allow use of the rental vehicle by any driver in violation of the requirements set forth above.Your immediate family members includes your parent, child, spouse or a domestic partner. In addition to the above, Additional Authorized Drivers who otherwise meet National's qualifications can be added at the time of rental. The Additional Authorized Drivers and you are referred to collectively in the Agreement as Authorized Driver(s).