How to update textbox on GUI from another thread

I'm new with C# and I'm trying to make a simple client server chat application.

I have RichTextBox on my client windows form and I am trying to update that control from server which is in another class. When I try to do it I get the error: "Cross-thread operation not valid: Control textBox1 accessed from a thread other than the thread it was created on".

Here the code of my Windows form:

private Topic topic;  
public RichTextBox textbox1;  
bool check = topic.addUser(textBoxNickname.Text, ref textbox1, ref listitems);

Topic class:

public class Topic : MarshalByRefObject  
{  
    //Some code
 public  bool addUser(string user, ref RichTextBox textBox1, ref List<string> listBox1)  
 {  
     //here i am trying to update that control and where i get that exception  
     textBox1.Text += "Connected to server... \n";  
}

So how to do that? How can I update the textbox control from another thread?

I'm trying to make some basic chat client/server application using .net remoting. I want to make windows form client application and console server application as separate .exe files. Here im trying to call server function AddUser from client and i want to AddUser function update my GUI. Ive modified code as you suggested Jon but now instead of cross-thread exception i've got this exception ... "SerializationException: Type Topic in Assembly is not marked as serializable".

Ill post my whole code bellow, will try to keep it simple as possible.
Any suggestion is welcome. Many thanks.

Server:

  namespace Test
{
    [Serializable]
    public class Topic : MarshalByRefObject
    {
        public bool AddUser(string user, RichTextBox textBox1, List<string> listBox1)
        {
            //Send to message only to the client connected
            MethodInvoker action = delegate { textBox1.Text += "C