I'd like to use organic group forums (og_forum) with a private group. I finally got it working, I thought... private forums were hidden from users who were not in the group. But there are two problems:
(1) When og_forum is enabled, normal forums are marked with the word "Private." There is an issue at http://drupal.org/node/202085
(2) The forums posts can be viewed by anyone who goes to "Recent changes," and they are emailed to everyone who has notification enabled, through the Notify module. There is an issue at http://drupal.org/node/244139