Very good question. You can use the "Group11" app to achieve your goal.
For the exclusive/internal contents, share them in "private groups" or "secret groups".
For the content that you want to be accessible for all your members, share them in "public groups" or simply share it on the home page, and do not assign it to any specific group.
Especially if you want the non-logged in users to be able to see content, the content should not belong to any groups, no matter what the privacy of the group is.