If you want to use grouping resources by departments in your organization, you can easily set it and assign departments to your users and resources. Departments can be defined on users or resources and later on, you can synchronize the departments' assignments between both objects.
To do add a department to a user, go to Setup.
Click on Users in the Administer → Manage Users section.
Department information is strictly connected to User, so if you want to add a new department in your organization or change the assignment of a user, select the User from All Users list and click Edit button.
Enter the Department name you want the user to be assigned to and click Save.
The department has been created and assigned or just assigned if it already existed. Now, the department assignment should be transferred from User to Resource data. Open the Admin page.
You can see the Synchronize button the number of synchronizations ready to proceed. Click on the Synchronize button to see them.
To see the details of synchronization click on the arrow on the left side of the Resource's name. It will show the differences between the Resource's and User's data.
To update the resource's data with user's data click the checkbox on the left side of the resource's name and then click ContactUser → Resource.
The department name will appear in the resource's data. Now you can assign also other Users/Resources to this department.