Hello,

Here is my situation:
I do not have any limitation as to the number of DB I can have

I have a master site called mysite.com with joomla 1.5.14 in /public_html

I also created subdomain site all with joomla installation:

community.mysite.com in public_html/community (using jomsocial)
forum.mysite.com in public_html/forum (using phpbb with Rockbridge to sync users)
jobs.mysite.com in public_html/jobs (using the templateplazza job board)

All have their own DB

The reason I did it this way is because I wanted to have xxxx.mydomain.com when someone access the forum or the community instead of the regular joomla component address .......index.php?option=com_xxxxx

The number one priority is to share the users across all domains. I want A SINGLE POINT SIGN ON and A SINGLE POINT REGISTRATION that will propagate among all domains.

I also want to show some module that are on the sub-domain on the main website. For example Jomsocial latest member module. Jomsocial is not installed on the main site, it is only installed in community.mysite.com. Would I be able to show the latest member module from community.mysite.com on mysite.com?

I saw an extension called SYNK in joomla.org, which is better to achieve what I want to do?

Thank You

Your question is not easy because you are using a bridge between PHPBB and Joomla.
I am trying to create an similar environment to reply to you but I am not familar with all those tools.

For the moment, my current analysis seems (conditional) to show that the bridge with PHPBB would use different plugin to synchronize the PHPBB extension that is totally independent.
So, share the standard Joomla users could help.
The problem that I have identified concern the replication of the Rockbridge plugins that are implemented as a core joomla plugin. Today the JMS tools is not able to install "core" extension. It just give the access to the normal extension. (But this may be changed in JMS to extend to the core).
Anyway, if you had to iinstall the Rockbridge several times, you could re-install it into slave site.

Concerning templateplazza I have not investigate.

With JMS, the user are really shared and not SYNChronized.
This measn that for standard joomla extension, the user information are present only once.
In the case of PHPBB, you will have a synchronisation between Joomla and PHPBB performed by the Rockbridge.

I think mixt different system (JomSocial), basic Joomla with PHPBB bridge could lead to problem in sychnronisation of all the websites.
I would recommend to use the same system to manage the user on all the website. (For example JomSocial) combined with the Rockbridge to perform the bridge with PHPBB.