One of the approaches are, You can make all the users subscribe to the SimpleChatModel and listen to the ChatEvent.HISTORY_CHANGE event. And in your event listener you could add your UI code to invoke the chat window at the receivers end.
Protected function init():void
{
_model = new SimpleChatModel();
_model.connectSession = _connectSession ;
_model.subscribe();
_model.addEventListener(CollectionNodeEvent.SYNCHRONIZATIONCHANGE, onSynchronizationChange);
_model.addEventListener(ChatEvent.HISTORYCHANGE, onHistoryChange);
}
protected function onHistoryChange(p_evt:ChatEvent):void {
//Invoke UI
}