You can't password protect any Nabble forum or sub-forum but the administrator can control which users have access to their forum or any sub-forum.
Read how to do that at:
http://greg-help.991552.n3.nabble.com/Controlling-Access-f4012840.htmlNote that the information is a little out of date as, for access control, the distinction between the "Anyone" and "Registered" user groups has been removed.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.