Click on the Watched Topics link at the top of the page. You can then select the appropriate forum topics to watch (such as JBoss jBMP). You will then receive emails when new entries are made in that topic.
There are mailing lists for JBoss available from sourceforge - all questions posted to the forums are e-mailed to the lists automatically.
The user list isn't really used for questions / discussions so it is better to just use the forums.