Redirects members-only content to the Membership Levels page on the Main Network Site if a user is logged out or not a member.

my_template_redirect_network_require_membership_access.php

<?php
/*
 * Redirects members-only content to the Membership Levels page on the Main Network Site 
 * if a user is logged out or not a member.
 */

function my_template_redirect_network_require_membership_access() {
	if ( function_exists( 'pmpro_has_membership_access' ) && ! pmpro_has_membership_access() ) {
		wp_redirect( network_site_url( 'levels' ) );
		exit;
	}
}
add_action( 'template_redirect', 'my_template_redirect_network_require_membership_access' );