Share the users between the website is something presented in the tutorial video 16.
www.jms2win.com/en/tutorial#jms12x-16
In the video 16, we show how to share the users between the slave 1 and its children sites.
In general, we do not recommend to share the users with the master website because we consider the master website as a repository or a library where all the extension are installed and without any content (empty).
If you want to share the users with the master, you need to create a JMS template rule where you define that you want to copy the "master" website and select in the "sharing" tabs that you want to share the users.
So the difference with video 16 is that you have to select the "master DB" website instead of the "slave 1" website in the JMS Template rule.
Template site = "master DB"
New DB prefix = {site_id}_
Sharing tabs / select the share the joomla users.