This should work (untested though):
code:
function OnEvent_ContactSignin(sEmail){
var ChatWnd = Messenger.OpenChat(sEmail);
ChatWnd.SendMessage("Hello (!N) !");
}
Lines of code should end with a semicolon (I think it's needed)
You were using a string as a boolean, which is possible, but not a good way of doing things.
Also, it shouldn't need to events... A ChatWnd object should be created and returned by the OpenChat function
Look in the scripting documentation for more help