In order to add 3 Additional Authorized Operators (AAO), the rental company wants an extra $24/day for the 10 day trip, and that's with a considerable discount.
I asked my insurance company what would would happen if I didn't add them to the contract and one of them was involved in an accident. The answer was:
"Well that is a hypothetical situation that I can not answer directly. All I can say is that from a policy perspective they would be covered, but from a legal perspective they might not be."
That was the best that I could get out of her since it was a "hypothetical situation".
So I called the rental agency and ran it by them. The rental agent was a bit more forthcoming.
"Your auto insurance policy coverage extends to any operator that you authorize to operate a covered vehicle and your coverage also extends to rental vehicles. Therefore, if you authorize someone to operate the rental vehicle, they would be covered by your insurance.
However, in a worst case scenario, if an operator who is not listed as an AAO with the rental agency has an accident, the rental agency has the option to consider the contract null and void, which essentially turns the vehicle into a stolen car. Since it would no longer be a rental vehicle, but technically a vehicle involved in a crime, it might not be covered by your policy."
He did mention that something like that has not happened, to his knowledge, in a long, long time, but in a worst case scenario, it could. He added that, hypothetically, it was up to me if I wanted to take that chance.