Select your Community and choose "Manage" the rightmost selection below the image.
Privacy settings can be customized using the box on the bottom right of the screen.
Community Page Privacy
From the "Community Page" drop-down you can select Public or Private:
Public Community - can be viewed by anyone in the company searching for Communities
Private Community - cannot be viewed by someone unless they are a member of the Community
Member List Privacy
From the "Show Member List" drop-down you can select Always, Only to Members, Only to Admins:
Always - member list will be visible to anyone who views the Community
Only to Members - member list will be visible only to members of the Community
Only to Admins - member list will be visible only to admins and not to anyone else whether or not they are members of the Community
Click the light green "Save changes" button at the bottom left of your screen.