add a priority pilot system
Many co-ownerships have a "priority pilot" scheduling system, where each owner gets a turn at being the priority pilot in rotation. These terms usually last a week and usually start and stop before Friday to include long weekends.
The priority pilot has unlimited schedule access to the airplane, and other pilots can reserve as well, but either a) the priority pilot must approve these reservations or b) the priority pilot can override these reservations, depending on how the co-ownership is set up.
There should also be a way for pilots to trade priority weeks, perhaps using an override interface a la PagerDuty. There should also be consideration to adding and removing people from the priority pilot rotation, and what to do about future reservations that shift out of priority weeks due to changes in the rotation.