Server:

+ Adding console with messages about connecting and disconnecting (e.g. 'ewa has left').
+ Adding 'Disconnect' contract to a server which removes client from a dictionary and sends message about his left to all users
Client:
+  Adding properly handled 'disconnect' function which calls server method 'Disconnect' in async mode

NOTE: From my experience (I've wasted some time on it...), if any new feature was added, serivce dll should be rebuild, readded to WPFHost which should be started and then new service reference from running WPFHost should be added to a client (address: net.tcp://localhost:7996/WPFHost/mex, Namespace: SVC; Advanced -> Generate asynchronous operations; Collection type: System.Collections.Generic.List). Last step note: In case of only update of already existing contracts, update service reference in client should be enough.