If you want to share the user, this can only be performed during a replication.
So create a fresh slave site with shared users is not possible because the "users" table must exists and can not be removed and replaced once a website is created.
So you have to create a JMS website template that consists in the replcation of the master website where in the "sharing" tabs you have selected to share the users.
This is moreless the same as in the tutorial video 16 except that you do not create a fresh slave site where you will store the users.
In general, I do not recommend to share the users with the master website.
You should better consider the master website as a repository or a library where all the extensions are installed and that is used to create the slave sites.
But you can do it.
So get inspired from the tutorial video 16
www.jms2win.com/tutorial#jms12x-16