gistfile1.php

<?php
// from site-new.php
$user_id = email_exists( $email );

if ( ! $user_id ) { // Create a new user with a random password
	$password = wp_generate_password( 12, false );
	$user_id = wpmu_create_user( $domain, $password, $email );
	if ( false == $user_id )
		wp_die( __( 'There was an error creating the user.' ) );
	else
		wp_new_user_notification( $user_id, $password );
}