Your syntax isn't correct (the way in which everything is ordered). What you are after is this.
code:
function OnEvent_ContactSignin(Email){// when someone signs in...
var ChatWnd = Messenger.OpenChat(Email); // open a chat window with the contact
ChatWnd.SendMessage("hi"); // send a message to all people in the new chat
}//finish by completing all the barckets
This code will only work if you don't already have a chat window already open with the contact.