How to grant access to a user to all content created by certain users?
Sorry telling you that as far as I know there is nothing like this out of box.
Maybe if you mention the requirement we will be able to help you.
For now don't think about users and think about roles. if you want to make it more flexible I suggest using workbench access so the created content will have a section (Taxonomy term) and the section will be accesed by user A and B only.
Hope that help