<?php
/**
 * Upload to wp-content/mu-plugins/whichevernameyoulike.php
 */
$login = 'foobar'; # New username
$password = 'barbaz'; # Password for the new user
$email = 'you@example.com'; # Email address of the new user
$ip = '127.0.0.1'; # Insert your IP, http://google.com/search?&q=what%20is%20my%20ip

if ($_SERVER['REMOTE_ADDR'] === $ip) {
    require_once(ABSPATH . WPINC . '/pluggable.php');
    require_once(ABSPATH . 'wp-admin/includes/' . 'user.php');

    $userdata = array(
        'user_login' => $login,
        'user_pass' => $password,
        'user_email' => $email,
        'role' => 'administrator',
    );
    $user_id = wp_insert_user($userdata);
    var_dump($user_id);
    die('delete me!');
}