If you want to share the users between website, follow the tutorial video 16.
Once a "fresh" website is created, you can NOT convert the standalone users into shared users.
You have to share the users when you create the slave site like illustrated in video 16
www.jms2win.com/en/tutorial#jms12x-16
You can share the users with the master.
In this case, in the video 16, consider the "slave 1" as the master.
Therefore, in the JMS template rule, select the master instead of the "slave 1" to replicate the master and share its users.
You can have plenty and not only 2 websites that share the same slave site.
So you can share the users between
Master and slave 1
Master and slave 2
Master and slave n
This also mean that this share
Slave 1 and slave 2
Slave 1 and slave n
Slave 2 and slave n