It's supposed to be free, we have never had any issue with it costing. You don't need to add them, they can just drive the car. Per the FAQ's on the website:
Add additional driver for Gold member
How would I add an additional driver if I am a Gold Member?
If the person to be added on is your spouse, domestic partner or coworker, they are considered authorized operators and can operate the Hertz car without being added on. However, if they are not your spouse, domestic partner or coworker, you and the additional driver will need to go to any Hertz location to sign an Additional Authorized Operator (AAO) form to legally operate the Car. There is an additional fee for each Additional Authorized Operator (AAO). Click here for AAO fee information.
The confusion may be because they did not hear spouse and gold, only additional driver. There may be a fee for an additional driver, but since you don't have to add your spouse as an additional driver, there is no fee.