You can make your community private so the content would only be accessible to logged in users. Additionally you can make the registration by invitation82, so only invited members can sign up.
Once you change the community access to "Private Mode", all pages of your community would be accessible only for the user who are logged in. Therefore, non-members are not able to access your community.
To activate the private mode, please follow the simple steps below:
Please do not forget to Save the changes.