Creating custom signup page for WordPress multisite

functions-part-2.php

/**
 * Return a value for '/signup'
 * Make sure that 'get_query_var( 'signup' )' will not return just an empty string if it is set.
 *
 * @link   http://wordpress.stackexchange.com/a/51450/22728
 * @link   https://www.pmg.com/blog/a-mostly-complete-guide-to-the-wordpress-rewrite-api/
 *
 * @param  array $vars  Query variables.
 * @return array        Modified query variables.
 */
function nanotuts_set_epex_var( $vars ) {
    isset( $vars['signup'] ) and $vars['signup'] = true;
    return $vars;
}
add_filter( 'request', 'nanotuts_set_epex_var' );

functions-scripts.php

function nanotuts_signup_scripts() {
      wp_enqueue_style( 'nanotuts-signup-styles', get_template_directory_uri() .'/css/nanotuts-signup-style.css' );
      wp_enqueue_script( 'nanotuts-signup-scripts', get_template_directory_uri() .'/js/nanotuts-signup-scripts.js', array('jquery'), '1.0.0', true );
}
add_action( 'wp_enqueue_scripts', 'nanotuts_signup_scripts' );

functions.php

<?php
/**
 * Rewrite endpoint : /signup.
 * @link https://gist.github.com/hasinhayder/c0f3968129a32df862a5#file-functions-php
 * @return void
 */
function nanotuts_registration_endpoint() {
    add_rewrite_endpoint( 'signup', EP_ROOT );

    if( get_option("EDIT_REWRITE_RULE") != 1 ) {
        flush_rewrite_rules();
        update_option( "EDIT_REWRITE_RULE", 1 );
    }
}
add_action( 'init', 'nanotuts_registration_endpoint' );

/**
 * Custom Template for Registration.
 *
 * @param  string $template Template to be loaded.
 * @return string           Our custom template to show up.
 */
function nanotuts_custom_templates( $template ) {
    if( ! is_admin() ) {
        if( get_query_var('signup') || strpos($_SERVER['REQUEST_URI'], 'wp-signup.php') ) {
            $template = get_template_directory() .'/signup.php';
        }
    }
    return $template;
}
add_filter( 'template_include', 'nanotuts_custom_templates' );

mu-signup.php

<?php
/**
 * WP MU Plugin
 * @author   Mayeenul Islam (@mayeenulislam)
 */

/**
 * Changing the Registration page URL.
 * @return string The URL of the new page.
 */
function nanotuts_signup_page() {
	$page = network_site_url() .'signup/';
	return $page;
}
add_filter( 'wp_signup_location', 'nanotuts_signup_page' );


/**
 * Redirect from default signup page to custom page.
 * @return void
 */
function nanotuts_redirect_from_default_signup() {
	if( strpos($_SERVER['REQUEST_URI'], 'wp-signup.php')) {
		wp_redirect(network_site_url() .'signup/');
		exit();
	}
}
add_action( 'plugins_loaded', 'nanotuts_redirect_from_default_signup' );