Multi-Site with Single, Shared User DB

Hello Drupal Community,

Forgive me if this is posted in the wrong forum. If so, please tell me the correct forum and I'll repost it there. Also, I've searched and searched and either my search-fu is no match or the results I do find are rather dated.

I've been assigned a project whereby I'll be creating multiple sites using the following configuration:

www.example.com
111.example.com
222.example.com
333.example.com

The main 'www' third-level domain will be the main page and host general information and whatnot. Subsequent third-level domains--and there may ultimately be many--will represent various regional offices; however, I want to create a global user database for all users as users will need to access a variety of regional office sites. Plus, it will be an absolute nightmare dealing with users with multiple logins to multiple sites.

So, not only do I wish to create a global user database, but, later on, I will be adding forums like phpBB and wish to use this same global user database to provide phpBB forum credentials as well.