This is one of the challenging aspects about defining permissioned spaces on atproto. In essence, you have a completely separate database per user (sits next to their repo) with which you can do permissioned public->private spectrum. Nesting more privacy inside another permissioned space requires breaking the typical permission walking chain, eg. in Google Docs, if you have access to a folder, you have access to the subfolders.
Yea, and it's a protocol problem more than anything. It would not be difficult to bolt on some kind of permissioning system to a PDS, but nothing else in the system would know how to handle that.
