I think it really depends on what you class as an intranet and what the user expect to do on it. It’s perfectly possible to do front end logins and display certain kinds of content to particular kinds of users based on role.
I would take a look at user kit, since this already handles alot of that stuff.
It is also possible to use Kirby for the content management side of things, and use it’s API to pipe stuff through to another system that is more geared towards an intranet via JSON endpoints.