The portal can't be completely hidden from guests, however any forum content that you've set for viewing by members only (categories, topics, gallery etc), will not be visible on the portal, just blank area's to keep such stuff private.
the portal can't be hidden for guests... I don't think there ever was an option like this. The forum can be blocked so even is a guest clicks a link on the portal they won't be able to see it till they register.
Go to "General / Categories & Forums" and click on this icon nearby a forum. Change the permissions to "Members" there. Do this for all your forums as well - if there is no board to view for guests on the index, the login screen will be displayed immediately.
The portal can't be entirely blocked for guests but you can hide single modules. In "modules / portal management / Structure" you can click this icon on already added modules and mark the boxes appropriately. This feature has been added recently with the new admin panel.