you should first build your own mechanism for notifying the Silverlight client that something has happened on the backend (e.g. a user has sign-in, or signed-off). One option for building this mechanism is to have a timer that will check for changes in the Database by using a web service. This web service can return the list of changed users, together with their status and then you can update the appropriate TreeView items to reflect the changes.
This way you will not need to refresh your application.
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