There are no departments in Mantis but so called access levels

Set the access level of users to developer for those users you want to see when assigning issues.
Set the acess level to vieweror reporter if you don't want to see a certain user when assigning issues.

xwR,

This is not a bug or feature request for MantisBT (you are asking for help on how to configure the system). I am therefore resolving this issue as "no change required".

Please use the forums to get support on customizing and using MantisBT (refer to http://www.mantisbt.org/support.php for links and further details).