Only one question: I want to limit the guestbook to registered user only to avoid spam. I set the Group for adding an entry to registered. However, I am still able to sign the book with any user name. The version is 18.104.22.168.
Thank you for your review, but...
...why don't you look into the support forum or ask for that?
For example here: http://joomla-extensions.kubik-rubik.de/forum/ebr-joomla-1-5/758-another-username-question