If you keep getting an overbooking message when making grooming appointments, make sure your options are set properly in File, System Options, Grooming tab.

You can either set the limit by groomer or by appointments.

If you choose by appointments, you need to set your max limit for each service (bath, groom, nails, others) on this screen.

If you want to maintain the max limit by groomer, click on File, Employees, Payroll Info tab.

Enter the max number for each service for each employee.  Be sure to hit Save after each employee.