As our community grows, we are exploring the possibility of sending users directly into closed categories within Discourse.
In other words, offering users a link to register and then directing them to a closed category during the onboarding process. Could we do that, and if so, what is the right process?
So private categories work off groups. So if you create an invite and choose a group which has permission to enter a private category your users will register and have access as they have automatically been added to the group.
as @ondrej said, if a new user registers with the invite link you provided, after sign up, they will be added automatically to the group you set in the invite.