Add All WordPress Super-Admins to a New Site, put file in wp-content/mu-plugins

add-superadmin-to-new-site.php

<?php
/**
 * Plugin Name: Add All Super-Admins to a New Site
 * Description: Will attempt to add all super-admins to a site when it's created.
 * Version: 0.1.0
 * Plugin URI: https://gist.github.com/soderlind/b0a7abc91eaa2646db38b81acfb532a1
 * Author: Per Soderind
 * Author URI: https://soderlind.no
 * Requires PHP: 7.0.0
 */

add_action( 'wpmu_new_blog', function( int $blog_id, int $user_id, string $domain, string $path, int $network_id, array $meta ) {
	$super_admins = get_super_admins();
	foreach ( $super_admins as $super ) {
		$user = get_user_by( 'login', $super );
		if ( ! is_user_member_of_blog( $user->ID, $blog_id ) ) {
			add_user_to_blog( $blog_id, $user->ID, 'administrator' );
		}
	}
}, 10, 6 );