Hello.
1) try to visit private (hidden) group's page as guest or registered user: crimson-land.ru/groups/viewgroup/64-zhest-dlya-vzroslykh
Result 1 (as expected): I can see only "Error. Sorry, you don't have permission to see this group". No group's header, avatar and "Join" button.

Hi,

This is valid bug, thank you for reporting.

cheers!

To fix this
- open this file components/com_community/templates/jomsocial/layouts/groups/miniheader.php
- add this code after line 22

if ($group->approvals == COMMUNITY_PRIVATE_GROUP && !$group->isMember($my->id) && !CFactory::getUser()->authorise('community.groupedit', 'com_community') && !CFactory::getUser()->authorise('community.groupeditstate', 'com_community') && !CFactory::getUser()->authorise('community.groupdelete', 'com_community')) {
     return false;
 }