I think you are asking, how a service responds to a client in a pub/sub service. If so, the answer is that the service uses the callbackchannel OperationContext.Current.GetCallbackChannel<IYourServiceContract>() that is supplied when the client "Subscribes". If you delve into this callbackChannel you will see that it maintains all of the necessary data to communicate with the client. If you notice, in the sample application you linked, the author is getting this from the person object, b/c he wants to store it in the chatters Array.
callback =
OperationContext.Current.GetCallbackChannel<IChatCallback>();
Here is an article I wrote that might simplify the pub/sub model a bit for you. http://www.codeproject.com/KB/WCF/wcfesb.aspx. It is not a chat program, but might explain a bit more about how communication is achieved through the callback channel.
If this is not what you are asking... please clarify.