Allow users to manage other users but all content is site-wide not based on group
This could be convoluted but this is how I would do it. 

To start with, I would use rules and have either entity reference fields or integer fields for the users and content. 

With rules, when the new users are created, USER A is referenced (or their UID in the integer field) using rules. 

Then, using rules, every node the new users create have this entity reference (or UID integer) applied to it automatically. 

For editing, I would give USER A's role editing permissions and then write a custom module that redirects users from the node edit page if the node's field doesn't match the user's field. (And allow certain roles like admin and also allow the node author).

One benefit of this is that you could create a view for USERS A that only shows nodes they can edit and provide a link per node. And I'd use an intreger field as a preference.

Edit: You could do all of this without any extant contrib modules or rules and just custom code. But I always have rules so it's an obvious go to for me.