Download (Import/Export in the menu) and update the User list column F with the departments the employees belong to and the department stats is activated. You will find the department stats under engagement in the menu.
Please note that any new groups added to column F will become a departments. You are not able to send invitations to departments, only groups, so if you need a mailing list that resembles departments, you both have to fill out departments and groups.
Employees can change department. This can be edited in the spreadsheet by inserting the new department in column F or edit user directly in Sociuu. To edit, go to Users and click on [···] and choose edit.