WordPress MU

Hello there,

I would like to add an extra input checkbox to the signup page to decide whether or not the one will get the notification of activation his account.

Does anybody know how to do that?

Thank you in advance.
James

However, I found on a line 1073 in a file wpmu-functions.php this which sends the email:

wp_mail($user_email, $subject, $message, $message_headers);

But how can I send to this file any variable from the signup form?