To assign Employees to a Manager you will first need to visit the Manager's profile, which you can do by visiting Employees in the top navigation and select View next to the Manager's name.
Once in their profile select "Employees Assigned" on the left hand side of the page.
Once at the employees assigned section, you'll be able to see the employees that are currently assigned along with the Manager's permissions. You can also edit assignments in this area as well:
The Approval permission gives a Manager full permissions over their assigned employees, such as editing their time and approving their PTO, while the View permission only allows a Manager to view information about the assigned employees.