Google Sheets does that automatically, though I don't know how "live" it updates the other open copies if you're updating something in a spreadsheet.

Office365 can do this too through Sharepoint (I love O365 Sharepoint), but I think it's less simultaneous-multi-user. You can use the web version of Excel to edit a spreadsheet that's online in Sharepoint, which as far as I remember locks the spreadsheet so that there are no conflicts.

Or you could write something in MS Access?
  (rar)

This is a common issue and has definitely been solved BY USING THE RIGHT FUCKING TOOL FOR THE JOB: I.E. A DATABASE WITH AN ONLINE GUI.

You should configure Exchange to block attachments for internal messages and go thump everyone who complains.

Also, everyone who recommends Sharepoint needs a slap too.

Sergiuiuiui is right about Google Sheets.  You can tweak collaboratively, with a live message feed on the same page.  If it's anything like Google Docs, then it updates in something like real time.