There is a “require login” setting if you want to be strict about requiring members to register.
But the only thing that will show is a “you need to login page” with no indication of what the forum is about.
The closest thing that I can think of to a “let them see but don’t let them see” tease is to create restricted categories and make a Banner post with links to them.
Visitors will know the categories exist but will not be able to get to them until they login, which means they will need to be registered first.