Group posts with privacy

How can an ES group remain an open group so that guests can see the activity of the group when browsing and still allow for the admin/members to post something that is only seen by the actual members of the group?

In the main activity feed, the registered-user can select to make their post public/registered users/friends but there is no option for that in the group's news feed posts. ....unless I'm missing some setting somewhere??

Thanks