If requirement is to show notify icon there must be an app or service running on user's computer which will wait for some message. Few days ago I already posted an answer to similar question on CP (link at bottom).
You can add some code to your app to wait for messages (app can be minimized to tray) or write another small app/service to handle messaging.
There is a windows network base solution but it shows Message box instead of notification icon. This solution is net send
Another solution is to send e-mails to users.
Here you have my answer to another question:
Notify icon to all employee computer's when adding new employee
I hope it help you.