When creating a community, you have three options for visibility: you can make the community public (everybody can see the posts in it), private (only members can see posts), and exclusive (the community is invisible to all users not invited). Once you create the community, you adjust the visibility only to make it more private. You can change it from public to private or exclusive or from private to exclusive. You cannot make the community more public. You are unable to change it from exclusive to private or public or from private to public.