Why get_users() not working on the admin backend?

Perhaps user.php has not yet been loaded. get_users() is a wrapper for WP_User_Query, which is defined in wp-includes/user.php. Hooks indicate template_redirect is after wp, after users are registered.

Perhaps you could try conditionally hooking ‘template_redirect’ on the front-end and ‘user_admin_menu’ on the back-end.

if (is_admin()){
   add_action('user_admin_menu','manage_new_user_submit');
} else {
   add_action('template_redirect','manage_new_user_submit');
}

Also, you might remove the trailing comma in that get_users() command. That can sometimes result in null.