The default settings allow "anyone" both to post and view a Nabble forum. Just make sure that those in the "anyone" group have the "View" permission set.
You need to be aware that at the top level you can either accept the default permission or select "Custom" and change it. At lower levels in the forum the default setting is to "Inherit" any setting from the parent forum. Again, you can choose a "Modified" setting, that over-rides the inherited one.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.