Hi,

Open your theme functions.php file and add following filter:

add_filter('stec_send_event_author_new_attendee', function($data, $event) {
    $organizers = $event->get_parsed_organizers(true);
    $headers = array();
    foreach ($organizers as $organizer) {
        if (false === filter_var($organizer->email, FILTER_VALIDATE_EMAIL)) {
            continue;
        }
        $headers[] = sprintf('Bcc: %s', $organizer->email);
    }
    add_filter('stec_wp_mail_headers', function() use ($headers) {
        return $headers;
    });
    return $data;
}, 10, 2);

With this filter the notification message will be sent to all event organizers as well.