Newly created Joomla website users (from Joomla or Community Builder frontend or backend) will be matched with existing CiviCRM contact(s) based on their email address. If no matching CiviCRM contact is found, a new one will be created and synchronized with the Joomla user. The plugin can also be configured to attatch and assign a new CiviCRM activity to the contact.
Once you install this Joomla 2.5/3.x plugin, just go to your Joomla Extensions -> Plug-in Manager and filter Select Type drop-down for user plugins. You will see the Joomla 2 CiviCRM User Sync entry. Click it to edit and read instructions on the left to configure plugin parameters on the right. Remeber to enable the plugin.
Note: This plugin will only work with CiviCRM 4.3.4 or better.