Redirect all non-logged-in users to the login page (private site). Add to functions.php.

WordPress Redirect all non-logged-in users

<?php 
// ----------------------------------------------------------------------------------------
// Redirect all non-logged-in users to the login page (private site). Add to functions.php. 
// ----------------------------------------------------------------------------------------

function admin_redirect() {
   if ( !is_user_logged_in()) {
       wp_redirect( home_url('/wp-admin/') ); 
       exit;
   } 
} 

add_action('get_header', 'admin_redirect');

?>