Surely you would have to enforce a policy that the aircraft is always topped up to the same level when it is finished with. That way, if person B turns up to find it below that level, he tops it up and sends the bill to the last person who flew it.
It's the only was a dry rate could work without a lot of guesswork on who has to put however much money in.
How many people are you intending to have in the group?