$aIDs = array();
$oInformationsystem_Group = Core_Entity::factory('Informationsystem_Group', 123);
$aInformationsystem_Groups = $oInformationsystem_Group->Informationsystem_Groups->findAll(FALSE);
foreach ($aInformationsystem_Groups as $oTmpGroup)
{
$aIDs[] = $oTmpGroup->id;
}
var_dump($aIDs);