Google Groups

Google Groups: "Choose an Access level
Public - Anyone can read the archives. Anyone can join, but only members can post messages, view the members list, create pages and upload files.

  Announcement-only - Anyone can read the archives. Anyone can join, but only managers can post messages, view the members list, create pages and upload files.

Restricted - People must be invited to join the group. Only members can post messages, read the archives, view the members list, create pages and upload files. Your group and its archives do not appear in public Google search results or the directory."