SignalR .Net client: How do I send a message to a Group?

What I have done with something similar is to create a method which accepts an object of your choice, e.g.

Your new class

public class MyMessage{
    public string Msg { get; set; }
    public string Group { get; set; }
}

Then create a method in the Hub that accepts this object.

public void Send(MyMessage message)
{
    // Call the addMessage method on all clients            
    Clients.All.addMessage(message.Msg);
    Clients.Group(message.Group).addMessage("Group Message " + message.Msg);
}

Then from your client, you can then pass this object in.

chatHub.Invoke<MyMessage>("send", new MyMessage() { Msg = "Hello World", Group = "RoomA" });

You can then also call this from the JS client

chat.server.send({ Msg: "Hello World", Group: "RoomA" });