There is a field that contains the username of the last user who logged onto the computer.  To add it, open one of the Computer Status Reports and select More Fields from the View menu.  Drag the field "Last Logon" onto the column labels row.  It will be added in front of the field that is highlighted by the dragging action.  A good location would be after the Client Status column.

To save the edits, choose either Save or Save As... from the File menu.