How can we share users between the master site and the slave sites? What about the default "admin" user? Will the admin of the master site be conflicted with the admins of the slave sites?

The sharing is decided when you create the slave site DB.
So it can not have any conflict with the slave site as there is no DB tables.

If your question is relative to how create specific privileges for specific slave site then the answer is that this is not possible.
When you share an extension, you share all the records of the tables that are shared.
Therefore the admin is the same for all the website that you share.

Thanks for your quick reply.

So how can we config the user sharing?

Also, after I setup the subsite through JMS and setup the proper subdomain, I go to the subsite installation page. And I setup the admin password during the installation procedure, which could be different from the master site. Then it means that for the master site, we have an admin password; for the slave site, we have another admin password. How is the admin shared between the two sites?

Review the tutorial video 16.
www.jms2win.com/tutorial#jms12x-16

To share the user, you can NOT create a fresh slave site like you explain. You don't have the "joomla installation page".

I repeat, when you share an extension, you share all the records of the tables that are shared.
Therefore the admin is the same (IDENTICAL) for all the website that you share.