You can assign the manager using the settings panel. 

1) Just click on this link and open the profile of the employee. 

2) Select the manager name from the dropdown menu and hit save. 

You have now assigned the manager to your employee. 

If you wish to change the manager, please type "Change manager @username" and select the name from the dropdown menu

Note:

If you wish to restrict your employee from changing their manager, you can do that on the settings panel.