The community wants to restrict access to discussions for unregistered users. joa searched for a global setting but couldn’t find one. bts suggested using the login required setting, which joa confirmed is the correct solution. Alternatively, ondrej suggested setting the forum to private in the setup wizard.
After enabling the login required setting, joa found the resulting start page to be unwelcoming and inquired about customizing it. featheredtoast provided the solution, directing joa to customize the login_required.welcome_message or welcome_message_invite_only text in the admin panel (admin -> customize -> text).