bbPress.org

By code – yes

any easy way – no

Can’t you use User Access Manager plugin?

https://wordpress.org/extend/plugins/user-access-manager/

If WP and BB are integrated? At least, that’s what I was hoping…

nope. a possible chance you can do somewhat with deep integration but that may be more efforts than its worth.

I think there is a plugin for that. Zaerl visibility does what you want perhaps. Search https://bbpress.org/plugins/

Use my plugin zaerl Visibility like Ash said.

This is a thing which is there in the bbPress plugin – private forums.