You can add employees both while creating and editing an a given schedule.
From the Work time side menu, select Work schedules. Create a new schedule by selecting ‘Create work schedule’ button or open an existing schedule by clicking the ‘Edit’ icon. If you don’t know how to add a new schedule, you can check it here.
Click the green ‘Add’ button on the right side of the page in the Employees section.
A list of departments and employees will appear in a new window – select the appropriate department or individuals and confirm with the green button at the bottom.