The rules surrounding additional drivers are a little more complex in North America, but there are many instances where, depending on the relationship of the driver to the main driver, they may be free.
California,
Illinois, Missouri,
Nevada, Oregon, and Wisconsin all have laws that allow spouses of the main driver to drive a rental vehicle automatically without being added to the contract (as long as they meet the age and driver's license requirements).
In California, rental companies are not allowed to charge additional driver fees at all, no matter the relationship any additional drivers have with the main driver. Additional drivers (aside from spouses and employees renting under a company contract) are still required to be present when signing the contract and must meet the same requirements as the main driver.
In other states and
Canada, spouses (and domestic partners in Canada) can be added to the rental contract at no additional cost with many rental suppliers (for example,
Enterprise,
Budget, and
Alamo). Other suppliers (including
Dollar and
Thrifty) charge for additional drivers that are spouses in states that are not noted above (except for members of their rewards programs).
In any case, you can see the additional driver fee for the car you plan to rent in the
optional extras section of the booking page (though keep in mind if that the fee may not apply to you based on the above).