Here's what I have so far:

IService:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.ServiceModel;

namespace ServiceLibrary
{
    [ServiceContract(SessionMode = SessionMode.Allowed, CallbackContract = typeof(IServiceCallback))]
    public interface IService
    {
        [OperationContract(IsOneWay = false, IsInitiating = true, IsTerminating = false)]
        void Join(string userName);
    }

    interface IServiceCallback
    {        
        [OperationContract(IsOneWay = true)]
        void UserJoined(string senderName);
    }
}

Service:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.ServiceModel;

namespace ServiceLibrary
{
    [ServiceBehavior(InstanceContextMode = InstanceContextMode.PerSession, ConcurrencyMode = ConcurrencyMode.Multiple)]
    public class Service:IService
    {
        IServiceCallback callback = null;

        public void Join(string userName)
        {
            callback = OperationContext.Current.GetCallbackChannel<IServiceCallback>();

        }
    }
}

Just a simple string passed from one client to another.