Public Space, Private Section

We have spaces that are public, but want to keep certain sections of that space private. I understand I do this by by adding groups, users, or teams to the visibility settings. Given the nature of this site, it is not practical to create groups for each project space, or add individual users to each private section. I created an administrative group, and use it to make the spaces private. This works for me much the same as the admin role. Anyone in it can see whatever they want, so I'm good with that. What I need is to add members to the parent space, and they get access to this now private sections. Am I missing something, or do I really need to either add the hundreds of parent space members to multiple private sections, or create an unmanageable number of groups to work around this?

Thank you,
Sean