Content-type linked to specific users and user-groups
Often times when using Drupal there are recipes or a collection of modules that will help you achieve your goals for a website so that your workload is  not burdened with custom PHP business logic code or SQL. Drupal helps to take away these technical difficulties when creating websites.

It sounds like you want to a site with groups of content and users. There exists active module(s) to allow easier site development of sections and groups of users within a website. The defacto module for this is Organic Groups (typically referred to as the "OG" module) and its set of sub-modules and contributed add-on 3rd party modules.


  Overview
  
  Enable users to create and manage their own 'groups'. Each group can
  have subscribers, and maintains a group home page where subscribers
  communicate amongst themselves.
  
  They do so by posting the usual node types: blog, story, page, etc. A
  block is shown on the group home page which facilitates these posts.
  The block also provides summary information about the group.
  
  Groups may be selective or not. Selective groups require approval in
  order to become a member, or even invitation -only groups. There are
  lots of preferences to configure groups as you need.


OG is not a small or simple module in it's administrative configuration, or it's supported API. But, it's goal is to make your stated problem easier to implement in a standard fashion/codebase for site developers.