The site Schedule displays all shifts created in the template. From there, you can then modify it to handle exceptions for any time range. Modify the Site Schedule when needed, typically for the upcoming week or two, and handle vacant shifts if your template includes any.
At that time, you will be able to modify shifts and add additional ad hoc shifts for clients that have requested them. The list of previously assigned employees will be displayed on the right-hand side of the screen. Pick and choose for the employees to fill in all available shifts. Perform this process while considering the compliance required for the shift.