Each site has its own php install, each with its own configuration.php file.

I have 1 data base, when installing joomla each time I used a unique database prefix for each site.

I then ran the scripts necessary to create the views for the 5 tables that relate to users shown below.

Finally I copied to each configuration.php file the secret code so they all match the primary site's secret code.

Then it all works.

So no, the websites are not using all the same tables. It does not make all the sites use all the tables in the database as I said each site has its own table prefixes, only the ones I mentioned above the 5 user tables share the same table data. Which is what the view handles.

This is quite important for me and I need to make sure I do it correctly.

You mention in the above reply to link the _comprofiler table at least. Is that table enough? - should I link others?

Below are the scripts that I ran to get the community site tables to use the primary site's user tables.

DROP TABLE `community_users`
CREATE VIEW community_users AS SELECT * FROM Prime1_users

DROP TABLE `community_session`
CREATE VIEW community_session AS SELECT * FROM Prime1_session

DROP TABLE `community_usergroups`
CREATE VIEW community_usergroups AS SELECT * FROM Prime1_usergroups

DROP TABLE `community_user_profiles`
CREATE VIEW community_user_profiles AS SELECT * FROM Prime1_user_profiles

DROP TABLE `community_user_usergroup_map`
CREATE VIEW community_user_usergroup_map AS SELECT * FROM Prime1_user_usergroup_map

I had the same request for 2 or more sites but the view function is very stressed and unstable for the databases especialy when there are joins.

The best way to use the same registration for all sites is to disable registration of the slave sites and link all relative links as "lost password" , "account manager", "registration" at the main site.
Main site che use CB queries plug-in to create the accounts in all sites.

Let me know if this solution work for you.