Money
Cost sharing
Splitting an aircraft's fixed and variable costs among the people who share it, under rules agreed in advance.
In practice
The durable pattern splits costs by their nature: fixed costs (hangar, insurance) divided equally because the airplane is equally available to everyone, and variable costs (fuel, reserves) charged per hour flown so heavy flyers fund their own wear. Groups get in trouble when the rule is implicit, retroactive, or lives in one person's spreadsheet - fairness is a formula best agreed before the month it applies to.
Get started
See it with your own aircraft.
Add your aircraft and your rates, then see the schedule, the squawks, and the ledger all work off one record. No card required.