protected function _initialize(KConfig $config)
{
    $config->append(array(
        'behaviors' => array(
            'assignable'=>array(
                'assignment_option' => ComComponentsDomainBehaviorAssignable::OPTION_NOT_OPTIONAL,
                'actor_identifiers' => array(
                'com:groups.domain.entity.group'
                )
            )
        )
    ));

    parent::_initialize($config);
}