Not sure if this is a bug or not, but
This is in 6.x-2.x-dev last downloaded Feb 5, 2010.
Users (in my case, anonymous users) without the "access content" permission are able to the view a list of users for any group, even if that group is a private group.
The view in question (og_members) has its page Access set to Unrestricted.
This was also brought up by YesCT in http://drupal.org/node/707738
Comments
Comment #1
claudiu.cristeaThis version of Drupal is not supported anymore. If this is still an issue in the
8.x-1.xbranch, please open a new up-to-date ticket. Closing.