I just checked on my local install and it seems to work correctly. When i create private group then it is not visible to other users anywhere. Same with the posts in private groups.

yes i made a mistake... but when you have the forum active in a group and you post something in that forum it shows up on your newsfeed so that all friends can see what you posted in the private group.

can you try that for me?

Even if another user is friend , still he/she can not see anything in regards to private groups on my newsfeed.Well, at least that is what i see on my local install.

Btw, you said "forum inside private group".What do you mean by that?

you can link the forum to a private group so that you can use the forum in a group. And there is the problem. 

Found a old topic http://www.oxwall.org/forum/topic/13977

there is the problem.