In my experience, no need to register a spouse/coworker as an additional driver.

Go to avis.com->Customer Service (top right corner)->FAQs->USA->Reservations->Rental Policies, Terms & Conditions:

The following individuals, if 25 years of age or older and in possession of a valid driver's license, may drive the car with the renter's prior permission and are exempt from Additional Driver fees:

Renter's employer (while on company business);
Renter's fellow employee (while on company business
Renter's spouse or life partner.
Disabled renters who have completed the non-licensed renter form


Late last year, we hit something in the road with a rental in my name and she wasn't listed as an additional driver. My wife filled out the accident form/signed it (and so did I as the renter) and they didn't question her being the driver.

It's also worth noting that Avis considers "life partner" someone with the same address on the driver's license. I've questioned them a few times on this (to be sure the policy hadn't changed, before my now-wife and I were married) but always get the same answer!