I used to rent out my TB20, years ago, dry.
It works provided that
- you have an accurate flow totaliser
- the fuel totaliser is always accurately preloaded with the FOB (easy if you always fill right up, which with a TB20 you usually do)
- nobody fiddles (fraudulently) with the FOB value
- one person is in charge of the invoicing
- each pilot reports the FOB reading before/after his flight, to the above person
Then it is pretty simple. The man in charge invoices each pilot for the hourly rate plus the fuel cost. If a particular pilot flies for an hour at 40 litres/hr then he will obviously get a smaller invoice than another pilot who flew for an hour at 60 litres/hr.
Normally you have an account at the local airfield fuel pump, and most filling will be done there.
If somebody fills up away from base, and pays for it themselves, then you have to make an appropriate adjustment.
One trick which caught me out was how to handle the duty drawback. In some shared-aircraft arrangements you get one individual who just happens to like flying the shortest possible foreign trip every time
If he is allowed to pocket the DD for himself, he will likely get totally free flying, at everybody else's expense. I had two renters (both being instructors) do this little con. There is no clearly fair way (to all) to deal with this which is also simple. I put in a scheme whereby I claimed back the drawback and then all those people flying afterwards got the benefit through lower fuel invoices, until the DD'd fuel was exhausted. Note that this issue is present in any group arrangement.
The advantage of dry rental is that it removes the common stupidity which you see all around airfields where the renter does everything he can (including power checks with a cold engine) before releasing the brakes and then races to the takeoff point.
The disadvantage of dry rental is the greater accounting issues, and having to trust people to not fiddle with the fuel totaliser (which one of my "instructor" renters did, not realising he would get caught by the EDM700 logs).
I gave up on all this hassle after a few years. Not enough quality customers.
Obviously forgetting about the totaliser and just filling to the brim (or tabs) is simple enough.