Before I decide to purchase the setup help, I figure I will give it the old college try myself.

If you can help me decide on the proper set up I would appreciate it.

I am building a national news network - with one primary national site, with many sub sites covering local markets. Each site will have the same template (with slightly different template settings like colors and logo), they will share users, content, componants, modules and plugins.

Currently I may be in a bit of trouble as the main network site came as an after thought - and is built as an add-on domain off /public_html/. the main site contained in /public_html/ root is a site that I will be recreating within the new network of sites (will be importing data etc). Is it possible to build this set up as it stands, or will I need to bring my other site down and rebuild from the root /public_html/.

Setup needed:

Primary national news site
- sub city site (sharing content, users and addons) (existing site content will have to be brought in via importing selective SQL tables)
- sub city site (sharing content, users and addons)
- sub city site (sharing content, users and addons)

With a template set so I can deploy a new city when I want as need grows.

If you expect to have plenty of local markets website, I suggest to follow the architecture described in the video 0.b - slide 21-24.

I suggest to create Addon Domain with document root like "domains/YOURDOMAIN.COM/public_html"
and deploy the JMS slave site into this directory [HOME_ACCOUNT]/domains/YOURDOMAIN.COM/public_html
www.jms2win.com/en/tutorial#jms12x-0b

Perhaps you could get inspired of the video 0.d that present a demo overview.
www.jms2win.com/en/tutorial#jms12x-0d

Concerning the sharing of the extension and some extension content, see the video 16
www.jms2win.com/en/tutorial#jms12x-16

I would suggest the following architecture on the disk

www.yourdomain.com -> domains/yourdomain.com/public_html
master.yourdomain.com -> domains/yourdomain.com/subdomains/master/public_html
cityX.yourdomain.com -> domains/yourdomain.com/subdomains/cityX/public_html
www.otherdomainX.com -> domains/otherdomainX.com/public_html

As you could see, each website would have its own directory and that could help you later to have specific images or templates directory for each slave site.
See video CP-05 that show how to have specific directories.
www.jms2win.com/en/tutorial#jms12x-CP-05

In addition, I do not recommend to put all the AddOn or Subdomain as a subdirectory of the "primary domain" to avoid it is published from this primary domain.

Concerning the DB and the content, get inspired of the video 16.
www.jms2win.com/en/tutorial#jms12x-16

Thank you -

For database setup - to share users and content, how do I do that?

Should I install all in one database, or use individual databases?

I currently have multisites installed on the network site, I did use the jos prefix - how ever I can execute an sql query to change that if needed.

I see you did comment on the DB, is that what is covered in that last video you mentioned

If you have some MySQL administration skills, you can try to setup your MySQL user privileges to have the permission to share the MySQL table users accross DB.

If you don't have a lot of MySQL administration skill, it is easier to do it in a single DB.

Remark:
If is NOT possible to convert an existing websites that already have independent users into shared users.
This is something that is decided when you create the slave site and not later.

the good thing is the only user that is created on the main network site current is the admin, and I can can create that on each site if needed.

Is it possible to import users once the system is set up, or does it require new registrations only.

If you want to import Joomla Users into a website, this can be done by an import of MySQL tables #__users but probably also all the #__core_acl_% tables.