If you have to revert to a legal arrangement them perhaps you should not be party to this arrangement?
I have been a member of a large group and the 'committee' spent most of its effort in trying to compile rules and regulations. (the PFA used to have an example document - it may still be available).
At the end of the day the aircradft will probably be grossly underused apart from public and school holidays. In a good group a simple conversation can sort that out. In a group with 'committee types' there will be rules and eventually it will be so difficult that the aircraft will be under utilised even for holidays!
If you know the people work it out. If you don't, try the rules, but the fallback is probably the internet sites.
The end result is acknowledge you are 'sharing' - that means everything will not necessarily go your way. The benefits are reduced cost, the disbenefits are that sometimes things do not work out, the aircraft is not available, not in the right place, not serviceable, not fuelled, whatever.
If you have to resort to the rulebook then it is probably more trouble than it is worth - unless you are ' a committee type'