Notify other forms when they close

Question

I have an application that I am developing, and what I'd like to do is to notify other people that are using the same application when they have closed the application.  Many people may be using this application, which is a windows form application, and I would like notify users  when other users have logged off.  I have looked into the observable design pattern, but since each application will have a separate a instance of the code, I am not sure how to implement this. 

Any thoughts. 

Thank you very much.