Vanilla offers a "Private Communities" mode to allow sites to remain visible only to its members. When under this mode, access to content and users is limited to only to those with an active account on the site. Anyone attempting to access the site without an account will be redirected to the sign-in page. Users will also be able to register or apply for an account on the site, based on the configured registration method.
Enabling Private Communities
The Private Communities setting can be managed from the top of the Roles & Permissions page in the dashboard. Clicking on the Advanced Settings toolbox and then "Enable Private Communities" will immediate toggle the "private" status of your site.
Registration methods on a private community work the same as a "public" community. If a site running as a private community is left in "Basic" registration, the intended privacy is lessened due to the open registration policy. Users still can't view the site without an account, but it is very easy for users to register an account when "Basic" registration is enabled.
Private communities will typically want to use a less permissive registration method, like "Invitation" or "Approval". This allows a greater degree of control over who is capable of registering. This additional level of control is beneficial to maintaining the intended level of privacy afforded by Private Communities.
Private Communities is an effort to increase the level of privacy available to users on a Vanilla site. However, it should not be treated as perfect. Depending on other site configuration settings, it may be possible for some information to leak. For example, user information is particularly vulnerable to discovery during open registration, due to necessary availability checks for usernames and email addresses.