The acknowledgements page shows that a lot of people have been able to contribute.
The jmri-announce mailing list carries announcements of new versions, major changes, etc. This list is intended to be very low traffic; discussions, etc, should take place elsewhere. You can subscribe by sending the word "subscribe" in the subject or body of an email to jmri-announce-request@lists.sourceforge.net.
We have a "JMRI users" Yahoo group for general discussion. If you want to ask a question, get some help, or suggest some improvement, that discussion group is a good place. You can subscribe, browse posts, etc, at http://groups.yahoo.com/group/jmriusers/.
Our technical web pages are still being written. This is still an early stage, and we're critically short of documentation. But if you're interested in working with the code, we'd love to help you get started via email, etc.