The default setting in a Nabble application allow anyone to view and post messages. Most people do not want this. They go to:
Options > Users > Change Permissions
and alter the defaults. If you forum is currently private then you have been there and changed things.
I am not clear what you expect from:
"anyone" to view the forum and force them to register"
In Nabble "Anyone" is a technical term, as is "Registered" and "Member". These are each "Groups" into which you can place any of your users and and restrict what they can do..
Take a look at this post and see if it helps:
http://support.nabble.com/Moderating-free-forum-tp6873076p6873871.htmlIf you have questions after reading it, ask!
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.