I am named as additional insured on a Warrior that is owned by the family of a pilot who died of Covid a few years ago. It was his pride and joy, and the family does not want to sell it, but none are pilots. So, I and one other local pilot fly it from time to time. The family pays insurance and maintenance, and hangar rent, and we just pay for the fuel when we use it. They have also invested in some improvements.
I also carry liability insurance for flying non-owned airplanes.
When we train someone in their personal airplane, we require the school to be named as additional insured with a waiver of subrogation. The only exception is for a Flight Review, where the exposure is pretty limited.