I am not sure how your chat code works, however I can suggest to use an interface with addMsg() method and have your clientObject class modified like below.
public interface MyInterface
{
void addMsg(string msg);
}
public class MyRemoteObject : MarshalByRefObject, MyInterface
{
}
public class clientObject : MarshalByRefObject
{
public Form f;
public clientObject(Form f)
{
this.f = f;
}
public void recebeMsg(string msg)
{
MyInterface obj = (MyInterface) f;
f.Invoke(new delChat(obj.addMsg), new object[] { msg });
}
}