When you create new users, you automatically assign them to a home department. You can also assign users to additional departments in Content Administration:
Click the Users entry under the department to which you want to assign the user.
Click the Assign additional users to this department icon in the toolbar.
Select the user's name from the Available users list in the Assignments section.
Click Add to move the user's name to the Assigned users list.
Click OK.
How do I assign a user to an e-mail queue?