$user1 = array('username', 'pw', 'mail',...);
$user2 = array('username2, ...);
$user3 = array('userdata...);
$addusers = array($user1, $user2, $user3);
$writer = XenForo_DataWriter::create('XenForo_DataWriter_User');
$writer->addMany($addusers);
$writer->save();