Require a group code to register for a level

my_pmpro_require_group_code_to_register.php

<?php // copy from below
/**
 * Only let level 2 members sign up if they use a Group code.
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */

function my_pmpro_require_group_code_to_register( $pmpro_continue_registration ) {
	// If there is already a checkout error, bail.
	if ( ! $pmpro_continue_registration ) {
		return $pmpro_continue_registration;
	}
		
	// if level = 2 and there is no group code, then show an error message
	$pmpro_level = pmpro_getLevelAtCheckout();
	if ( $pmpro_level->id == 2 && empty( $_REQUEST['pmprogroupacct_group_code'] ) ) {
		pmpro_setMessage("You must use a valid group code to register for this level.", "pmpro_error");
		return false;
	}
	
	return $pmpro_continue_registration;
}
add_filter( 'pmpro_registration_checks', 'my_pmpro_require_group_code_to_register' );