As you assumed – the solution is pretty easy. First create a file(for instance) skip-email-check.php and inside it put the code from the other answer: add_filter(‘wpmu_validate_user_signup’, ‘skip_email_exist’); function skip_email_exist($result){ if(isset($result[‘errors’]->errors[‘user_email’]) && ($key = array_search(__(‘Sorry, that email address is already used!’), $result[‘errors’]->errors[‘user_email’])) !== false) { unset($result[‘errors’]->errors[‘user_email’][$key]); if (empty($result[‘errors’]->errors[‘user_email’])) unset($result[‘errors’]->errors[‘user_email’]); } define( ‘WP_IMPORTING’, ‘SKIP_EMAIL_EXIST’ ); return $result; … Read more