I'm not totally sure if I understand all your requirements correctly. But it seems, first of all you need the possibility for portal users to be part of different groups. And the possibility to show/hide blocks, links and pages according to their group.

This module https://apps.odoo.com/apps/modules/16.0/website_user_types provides this option. 

In Detail: Users, that register themselves would be part of Group B2C. And you could set users, that you create manually in the backend, to B2B. Recently, the module was extended to allow more groups than just these two.

Depending on your use case, the whole scenario might require some configuration.