Display username in message box that has a file open

I have bit of code that:
(1) opens another spreadsheet
(2) updates that spreadsheet
(3) saves that spreadsheet
(4) closes that spreadsheet

If the spreadsheet that requires updating is open by another user, I display a message indicating the file is open and no updates occured. There are about 20 persons within the organization that may have this file open? Is there away to display the username of the person who has the file open? Thanks in advance for any comments.