You can fix this by reading the message, but more code is required.

You have decorated one or more objects with [MessageContract] instead of [OperationContract]. When you use this attribute you no longer pass individual parameters but the whole object instead. Please replace MessageContract with OperationContract or only pass in the class that has the MessageContract attribute

 

  
public void AddUser(Recipient recipient)
{

}

[MessageContract]
public class Recipient
{

}