The company I flew with had to put me on the books as a volunteer employee in able to fly in the aircraft by their manual. I also had to sign a contract saying that I would not hold them or the insurance company liable in case of an accident. The captains were both ATPs and the flights were logged as training, dual received, since there was the possibility of a position coming available for a single pilot VFR 135 in the future. Unfortunately the SEL pilot position never came up because of the economy right now.