Managing group access for a Joomla multisite

Again, this is a manual configuration of a seemingly multisite feature using OpenSEF. This means that I only have one Joomla installation but many subsite pages which is done by changing the SEF URLs of the site. This also means that you have one Joomla installation consisting of multiple sections for each of your subsite. So when you enable OpenSEF, you can just set Section/Content-Title.htm for the URL. that way it looks as if the subsite is a separate CMS from the main site. Egad, different templates, and main menus. So the problem is:

How do you set up this whole system where users and content providers from these subsites can only add/edit/publish content on their respective subsite sections??? Is there a feature to add a group access on certain content pages?

So I'm now looking into JUGA Joomla User Group Access.

You can download it here but NOTE that you have to register to their site first.

I'va already looked at it. The problem is that I can only create group access on components, contents, and modules, but not categories or sections. I would have to assign e every page created, to a certain group.

Why not assign a group access to a category instead??