This projects offers extensions for Joomla and Openfire that allow their integration.

OpenFire Joomla Integration Plugin

OpenFire Joomla Integration Plugin allows the OpenFire XMPP (Jabber) Server to authenticate against the Joomla user database. This allows members of your Joomla website to have Instant Messenger accounts at your domain.

See: http://www.fijiwebdesign.com/products/openfire-joomla-integration-plugin.html for documentation.