Administrators can add department administrators in two ways: one, from the Organization Chart which can be accessed through My Space; two, from the departments section which can be accessed through Settings. On the Organization Chart, go to Departments. Here a list of all the departments in the organizations are visible.
When you click on a department, it expands and you can all the users who are a part of the department. To make a user a department administrator, just click on the ‘Mark as dept. Administrator’ icon.