Agree on the basics
When more than one person uses a vehicle, agree how bookings and changes will be communicated. A shared calendar is useful only if everyone knows where to look.
Include practical details
Add the expected return time and any arrangement that affects the next person. Keep personal addresses and other sensitive information in a private channel.
Review the handoff
Before handing over the car, communicate relevant changes or concerns. Use the calendar for scheduling, while keeping mechanical questions with qualified professionals.