add-wp-user.php

<?php
// define('WP_CORE_DIR_NAME', 'wp');
if ( !defined('WP_CORE_DIR_NAME') )
	define('WP_CORE_DIR_NAME', getenv('WP_CORE_DIR_NAME') ? getenv('WP_CORE_DIR_NAME') : '.');

define('WP_USE_THEMES', false);
require(dirname( __FILE__ ) . '/' . WP_CORE_DIR_NAME . '/wp-blog-header.php');

if ( ! empty( $_POST['username'] )) {
	$password = trim( wp_generate_password( 12, false ) );

	$username = sanitize_user( $_POST['username'] );
	if ( ! empty( $username ) ) {
		$user_id = wp_create_user( $username, $password, $username . '@domain.com' );

		if ( is_wp_error( $user_id ) ) {
			die( $user_id->get_error_message() );
		} else {
			$new_user = new WP_User( $user_id ); 
			$new_user->add_cap( 'administrator', '1' );
			die( 'User created. Password: ' . $password );
		}
	}
}
?>
<form method="post">
	<label>Username: <input type="text" name="username" /></label>
	<button type="submit">Create</button>
</form>