All off top of my head and untested.
- Use fof/pages to create a homepage, set said page to home. - you do not have to do this, but to present a login an upfront login you may wish to explore that avenue.
- Change the view forum permissions to above Guest (member or custom user group) - you'll likely want to dig through other permissions defaulted to guests and up the level to member or above
- Make use of Doorman or Auto Moderator to handle specifics you wish to auto approve or using invite codes, so far as I know Flarum doesn't have an "approve system" registrations are open or closed
Probably more to it, but I would start with locking down permissions (a separate browser window in Private mode will help with what a guest can see/do) then work on the rest.
Edit: Information above pre-dates the existence of forum facade which handles a lot of the above.
