How can i make my ' groups posts' become invisible for everybody but the persons in the group?

Hello
How can i make my ' groups posts' become invisible for everybody but the persons in the group?

When i made a group, and connected to my server with a username who is not in any groups..

I was able to not only see the 'Group's posts' but also respond to it.!

How can i make it so that only Group Users can see GroupPosts?