Franz Kössinger has just released PassPhrase for CB intended for closed groups ie you email the passphrase to invited guests. The passphrase is needed to submit the registration form.
I changed the info text in the passphrase field (in cb.passphrase.php) to ask a simple question. The answer is your passphrase set in the plugin.
I tested it with CB 1.1 using Joomla 1.53 and it works! The field entries are not lost either if a user enters the passphrase incorrectly.
