You *must* have some server-side code (PHP, ASP, JSP, etc.) that receives a message (presumably via AJAX) from one client to store the changed information. Then the Admin page can "scan" (again, using AJAX, and making the request every few seconds) by asking the server what has changed.
An optimist sees the glass as half full.
A pessimist sees the glass as half empty.
A realist drinks it no matter how much there is.