When you go to your group you see the text i mention above.
When you click on Manage group you get another screen where you can write something about your group.
Then you can choose who can see all the posts. All users - Only group members
And then you can choose how people can join the group. Free - Premoderation - Invitation - Close.
I add a screenshot of all the options after you clicked on manage group.
chekccard
