Chatango is an online web-based chat service, with many different features and new ones being added all the time.

One feature that is strangely lacking however, is private groups, groups that only certain people can enter.

Chatango allows you to add moderators and admins to your chat groups, and modify what actions each is allowed to perform. A Private Group feature could probably be done very similarly, with an extra options menu to decide what default users can do (view posts, make their own posts, view the mod list, view the user's list, etc.), and an extra menu similar to the moderators menu to add certain users that would have their own settings.

With this example, if you wanted an "invite-only" group, you could set the default permissions to nothing, so unspecified users could not access your group. Then choose specific users, and give them permission to view and create posts.