Hi All,
In one hand I agree that a water tight contract is the answer, but I have to say that when I bought my very first share in an aircraft it was with 4 people that I had never met before. There were no contracts, no written rules, no nothing and for the whole time I was in the group, there was never a wrong word between any of us. Two of those original shareholders are still very close friends to this day (not that I've fallen out with the other two).
I would say that if a shareholder reneged on their part of their obligations, then you should suspend their access to the aircraft until such a time as they have brought their arrears up to date and/or dispose of their share and deduct the arrears from the sale price. This would obviously be an extreme scenario, but any rules you have should provide for it.
Joel