... in Joomla and there is no such user in Joomla, the bridge tries to authorize this user in phpBB and next auto-create and auto-login such user in Joomla on success.

And is it possible to make a selection of registered BB users (member-only) before migration??

No. You can only copy all phpBB users.

Hello,
I already have a Joomla website for my club and a PHPBB bulletin board on the website working.
At this time club members are not registered users of this Joomla website, but most of them are registered users of the bulletin board in order to see attachments of most posts.
Since I am thinking to ...

... in common is the SHARING OF USERS. I mean if an user is registering on an website (no matter if master or slave, can be anyone) to look like is registered to rest of sites; I mean if the customer click on "My account" or My Profile, whatever, to be loaded his info from one single table from main ...

For Example, I have a site master.com and (slave.com or slave.master.com or master.com/slave)

master.com is a new/magazine site - I want registered level users to submit just an article or news but won't be able to submit videos
slave.master.com is a Media sharing website - I want all registered user ...

Yeah, we want the users to be registered on all sites, but not necessarily show up in search of maps or such. Sounds like we need a separate registration field for "active sites" which we check for when displaying search results, something like that?

Useful feature and important compatibility fix.

• Feature/Fix: allow to sync 'Newly registered users' phpBB group on phpBB 3.1.
• Fix: Joomla 3.4.6 compatibility

Read full article...

Each site can have own template style in Joomla as usual if each site uses own database tables.

User A registered on Site A
→ User's record is available on Site A or B for login

What happens of user B registers on site B or C...?
If users are shared - user record is available on all sites which ...

... about how ACL is managed though. You mentioned above that MightySites doesn't sync or replicate database data.

is this cirrect then:

User A registered on Site A
→ User's record is available on Site A or B for login

What happens of user B registers on site B or C...?

WHat happens if the user ...

... native Joomla database layer).

So any user-related action on any site will affect all sites because same tables are affected.

Once a user is registered on any site - he can login on any site.
Once a user is deleted on any site - he is missed on all sites.
etc.

In either case I expect that any ...

... of site B for users, so both sites will use same database tables and hence all users will be always same on both sites.

No matter where user is registered/updated/deleted - this action will affect both sites.

If 2 sites have same users - we can also setup special single login, so user logged on ...