function modname_form_alter(&$form, $form_state, $form_id) {
    switch ($form_id) {

        // This is our form ID.
        case 'user_register_form':
            unset($form['#groups']['field_group_name']);
            unset($form['#fieldgroups']['field_group_name']);
                break;
    }
  return $form;
 }

field_group_hide_field_groups($form, array('field_group_name'));

hide($form['#fieldgroups']['field_group_name']);