I co-lead the London Java Community (LJC) aka the London Java User Group (JUG). We recently (last week!) were the first JUG to be elected to an open seat on the Java Standard Edition/Enterprise Edition Executive Committee (Java SE/EE EC in short).
Become a JCP member
You can become a JCP individual member very easily and you can also join as part of a corporate, academic, non-profit or JUG organisation. This is the first step you should take to get involved. It's actually very easy to join, see the JCP home page for instructions
It's not as easy to get involved in a JSR as we'd like
Currently it can be quite difficult to get involved in the average JSR. Under existing rules, parts or even all of a JSR can effectively be run in private, making it impossible for outsiders to join. Most JSRs run at least partly in the open, but several don't.
There is also a tendency to come up with a Technical Compatibility Test (TCK) and Reference Implementation (RI) quite late in the piece, which doesn't allow the wider community to actually 'play' with the proposed JSR and give meaningful feedback.
Some JSRs are simply just deeply technical and difficult and only real experts can get involved early on, but that's just the nature of the beast of something like JSR-292 (invokedynamic byecode for the JVM).
But you should still jump on in
That said there are several JSRs which are run in the open and do solicit feedback with early RI's and TCKs. Please visit the JCP home page and browse through the JSRs on the left hand menu. Each JSR page will list their public mailing lists, issue trackers etc. Simply join the mailing list, say hello and ask how you can help out (even though you're not necessarily a domain expert).
JSR-107 (Caching) is an example of a recently revived JSR that's running out in the open and is happy to receive help (big and small) from Java enthusiasts.
Things are about to get a lot better!
This is a massive time of change in the Java ecosystem and during times of change you have the best chance to positively influence the outcome.
Oracle is working very hard to make the JCP and JSRs more open. Despite much anti-Oracle publicity, they really are trying hard (see JSR-348 comments below). Sure, there's still plenty of areas that we'd like to see Oracle change (and we'll be advocating for those), but they do engage in dialogue and seem genuinely willing to listen.
For the first time, two JUGs are on the EC (us & SouJava - The Brazilian JUG). This means that the world wide developer community (9-10 million) has direct representation for the first time
JSR-348 has just been announced which is going to take great strides to open up the JCP, the Expert Groups (EGs) and just the overall ecosystem of standards. We implore you to get involved and send in feedback, whether its to us, your local JUG leader or through hte official JCP channels (see the contact us on at jcp.org)
The LJC and many other EC and EG members are very firmly in the camp of making JSRs more accessible to everyone. As well as enforcing openness via JSR-348, we also see a very real chance to have each JSR really engage with the community. We're going to try and work with JSR EGs to see how we can raise their profile, make them really easy to access etc. Something along the lines of running a successful open source project is what we're looking at.
Phew, long answer, but I'm really excited about the future :)
Martijn (co-leader LJC - @karianna)