1. Open the Dashboard, click Sharing
  2. Click the double arrows to allow access to every user
  3. Go to Administration, Set up System Security and open the user record
  4. Click the Account Security tab and select Allow access to selected Accounts from the drop down menu
  5. Click the radio button next to Query, then click the binocular icon
  6. Click Add a new Query to create a new account query
  7. On the criteria tab expand Account, Account Segments, double click the segment that defines department
  8. Select equals as the Operator and the desired value
  9. Save the Query
  10. Save the User record

Note: This will restrict the user's rights to only the selected accounts throughout the system