The aircraft is always left with the same fuel level for the next member.
Lots of problems that you need to think about if you implement this. If "the same level" is set too low then you end up pushing VFR/IFR minimum fuel limits, and you need to find a way to charge if the level ends up too high because of an abandoned flight (wx related for instance). If "the same level" is set too high then you might run into W&B issues.
Furthermore, you end up refueling at least once per flight. Even though the tanks might hold sufficient fuel for four local flights before refueling would actually be needed.
A "wet" hire rate as Ghengis suggested is far easier to implement. My club actually pays back fuel bought elsewhere *at the home base rates*. This allows the club to forecast fuel costs easier (since all fuel costs the club the same) and encourages the hirer to shop around: the hirer pockets the difference.
Disadvantage of a "wet" hire rate is of course in any case that it doesn't encourage the hirer to apply proper engine management. Leaning, most of all.