Allows to add new fields to the Create/Edit Product Group form in the Administration Area. 
Usually this event is used together with the 
shop:onExtendCustomGroupModel event. 
To add new fields to the product group form, call the 
add_form_field() method of the 
group object.
public function subscribeEvents()
{
  Backend::$events->addEvent('shop:onExtendCustomGroupModel', $this, 'extend_custom_group_model');
  Backend::$events->addEvent('shop:onExtendCustomGroupForm', $this, 'extend_custom_group_form');
}
public function extend_custom_group_model($custom_group, $context)
{
  $custom_group->define_column('x_gender', 'Gender');
}
public function extend_custom_group_form($custom_group, $context)
{
  $custom_group->add_form_field('x_gender')->tab('Group');
}