Sure, you can merge database tables of users (manually via SQL queries).
The main problem is that all users references should be also updated in all related tables.

"It's not a problem for newly created slave sites."

You mean that we can somehow merge the new users of the new save sites into the Master ?

Thsnk you

If you want to share users - all sites will have all same users from master site. So existing users from slave sites should be copied into database of master site if you want to keep. Thought note that users IDs will be changed and all users relations will be broken. It's not a problem for newly cre...

If they gone , how new users in a slave site,will have this option ?
To login in the master or other slave site?
Single frontend login/logout in specified Joomla! websites if login/logout on website with the extension installed.

No, all sites will have users of master site.
All uses of slave sites will gone. You should merge them into database of master site manually.

Initially all sites will have current users of master site.

And then all users from all sites, correct?

All sites will use same database tables for users (from database of master site), so no matter where user is registered/created/updated - these changes affect all sites.
Initially all sites will have current users of master site.

When someone register in a slave site,
he exists in master site ?
To understand me
i want to use 1-2-3 slave sites so i will make the master site bigger (more members)

If you want to share users - yes, all slave sites can load users from database tables of master site.

All sites should use same MySQL server and MySQL users of slave sites should have access (full privileges) to database of master site.

I hope here is the correct place for posting my questions.
I want to first ,
we have sites a,b,c ,
and DBs a,b,c or only the db of a site?

Can only one be common DB for members ?
Thank you