Popup window when the Excel file is saved/updated

Hello,

I have an Excel file which is used by multiple users (saved on the network drive). They can only open the file as read-only.
I'm looking for a macro that will create a popup window (at the computers which have the file opened - read only) whenever I save the file. So that they know the file has changed.

I have searched on google for a while and found this thread:
https://ccm.net/forum/affich-107579-auto-notify-or-mail-when-excel-sheet-updated
The code mentioned in one of the answers lets me send an email. I've tried to adapt the code but so far no success.

Can anyone help me with this?
Thanks in advance!