Hi, I've a Master site which contains the users database, jomsocial and a phpbb3 forum. Then I've a Slave site which is suppose to have it's own articles and social network. However, I want the user registration to store the data only into the Master site so that there is only 1 user database that serves all the Slave sites' jomsocial social network. Users should also be able to access the phpbb3 forum in the Master site after they log in.

I have already done up the Master and Slave site before I bought this component. How do I go about to share the user database with the Slave sites? And since I can't do a new installation for the Slave site that I have, how do I define it as my Slave site? Pls help! Thanks!

Share the user with JMS is only possible when you create a new slave site.
When you already have a website with independent users, this is not possible to convert independent into shared users.

The reason is a problem of consistency because in the current website database you may have extension that are currently using the users with a specific ID. If you decide to move the existing users into another website (DB or table prefix), this mean that the ID will be modified and that you should update all the tables of all the extension to restore the consistency.
JMS does NOT provide such very complex operation.
This is the reason why we have written that the user sharing or extension sharing is only possible when you create a new website. This can not be change after the creatiion of the slave site DB.

i see. thanks for your explanation.

If I create a new slave site in a new installation. After the installation, can I copy the files I have from my existing site so that I do not need to redo the site? I understand that I will have a clean user database now and the users have to re-register to the site to work.

YES.

Sorry if we can not provide you a magic solution.

Becarefull when using PHPBB3 because it saves its data with its own prefix.
The reason is that this is an extenal extension for which a bridge is created with Joomla.
So if you want to have separated PHPBB3 forum, they probably have to be located on different DB.

The phpbb3 is bridged to the Master site and there will only be 1 forum. If the user databse from the Master site is shared across the Slave sites, I supposed it should be quite safe?

I hope also but I suggest that you check it yourself with a small prototype to see how the users are shared between different website and with a same PHPBB3.
I have already tried configure a such PHPBB3 environment to perform this kind of test but unfortunatelly, I don't know how PHPBB3 is working and I have not time to learn this product.

Up to you to perform the test as you have all the element to check it and you already know how works PHPBB3.

When done, I suggest that you post a message in the forum to explain the procedure and give your feedback