I want to make an Organic group private,to be seen only from members of this group. All the other will get an "Access denied" or something like that. For the time being,I can see the Group but not its content. I would also want NOT to be able to see the existance of the group,if I am not a member of it. I am working on Drupal 7. How to do that? Thanks
You need to enable og access control module.
Then navigate to admin/config/group/fields, add an visibility field to your og bundle
Then edit the og bundle content type, there is a new field added to it, make your change for public or private
Please consider that to make a group truly "private" you have also to remove the "register" permission for non-members otherwise even if the group is "private" non-members that can guess its nid can subscribe (at http://www.example.com/group/node/[nid]/subscribe) and hence view the group's content [OG 7.x-2.2].