User authentication from an existing database

Hi everyone. I'm looking to use XWiki for our corporate intranet. I would like to authenticate users from our existing database, like it says on this page: http://www.xwiki.org/xwiki/bin/view/AdminGuide/Pluggable+Authentication It says "Users can be authenticated against custom tables." Can somebody tell me how to do this? Or do I need to write some custom code? Thanks. Sorry if the answer is documented somewhere; I haven't been able to find it. -- Tim K