In this tutorial, you will learn how to set up your site so you only need one set of users for multiple Joomla installations, as long as they all reside on the same database server. The original idea came from birnik, who wrote a tutorial on how to do it in Joomla! 1.5. It would help if you were familiar with phpMyAdmin to complete this tutorial.
Warning: Please make a backup of your site before trying these steps.
Also be very aware that all users including Super Users will have access to all these sites you set this up on.
Do not trying to restore a slave site to a different server, separated from the master site, without restoring your views into real tables.
The only scenario you should be using this method is if you are building one main site with multiple installs of Joomla.
Create a View for each table you have dropped, as shown in the above query.
What this does is takes an instance of all these tables from site1 and creates a view of the data. So your "site2" thinks all these tables exist, but it is just an instance from your "site1". This is how the "synchronizing" takes place. :-)
You can now login to your master site and add users. All these users will be able to login from all the other sites you have set this up in.
Please note, to prevent issues on your slave sites, it may be good to redirect registrants to the master site to register instead.