When all you have is a hammer, it's the tool you always use.
That to say, apologies for always defaulting to Drupal, but as I know it best, it's the one I can think of.
The Organic Groups functionality of Drupal is more or less for this purpose. You would just create regional groups and people would be members of the site and then as many groups as they are members of.
It is the software running Drupal Groups which has thousands of groups.
They should be able to do that, though realistically I think any time you send out a newsletter from your own site, you can run into deliverability issues. So it's probably better to try to plug into Mail Chimp or something.
Sounds to me like a WordPress multi-site installation would work. You can control the themes to be similar, yet allow each group to have their own membership with their own unique logins. This way each would be able to add as much (or as little) to each of their areas as they wish.