According to the rules of Emerald Club, spouses (and any immediate family members) can drive the car at no extra charge. It doesn't say anywhere that you have to go to the counter to have them sign the agreement. It is part of the EC agreement.
This is copied from the Emerald Club info at nationalcar.com
Automatic Authorized Drivers
Free (in the United States and Canada) An Emerald Club members spouse, immediate family member, business partner, employer, or fellow associate who has an assigned corporate contract with National and 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.
Free (in the United States and Canada) On commercial rentals, the renter's business partner, employer or fellow associate, who has an assigned corporate contract with National and drives the car for business purposes.
All other additional authorized driver situations are subject to a daily charge.
Paul