There is usually a charge for additional drivers, unless:

-The rental originates in a state that by law says spouses are allowed to drive for no extra charge.

-It's an association or corporate rate that says a spouse, coworker, other AAA member, etc. is allowed at no extra charge. Hertz Gold, AAA, and AARP allow this, I believe. For "discount" brands, the rules might be more restrictive. Check the T&Cs.