The comment gatekeeper is a lightweight plugin that adds a simple question and answer to your comment form. The comment form displays the question and the commenter types in their answer as part of the comment form. If the commenter answers incorrectly, then their comment fails and doesn’t get added.
The primary use of this plugin is to reduce comment spam but it can also be used to make your comments “by invitation only.” To do this, you can create a question like, “Please enter the invitation code” and then email your invitees the code. In this way, only those who have the invitation will be able to enter the code and comment.
I’m looking forward to releasing the plugin and also hearing feedback from people about its operation. If you are interested in trying out a pre-release version of the plugin, just leave me a comment here and I’ll follow up with you.