1) A user subscribes himself to Plan A and gains the Access Level "Publisher".
CBSubs can certainly add usergroups based off subscription.
2) once the user is login, he can create other new users with Plan B and access Level "Register" and automatically connected.
That's not doable with anything we provide unless you implement it your self using something like RS Forms, Breezing Forms, etc.. and maybe CB Auto Actions with its Registration action as the endpoint for your custom HTML form. As there is no frontend user create beyond normal registration workflows.