Open source projects are made of contributions. Anyone can get involved by submitting patches/pull requests to the code, documentation or anything else. Reporting bugs and explaining with a good level of detail how to reproduce it is extremely important.
If you not familiar with patches, see How to submit a fix or a new feature article.
Just like The Apache Software Foundation, we are also under the philosophy of Meritocracy:
The roles and responsibilities that people can assume in the project are based on merit. Everybody can help no matter what their role. Those who have been long term or valuable contributors to the project obtain the right to vote and commit directly to the source repository.
After a demonstration of commitment, good technical skills and more important good judgment, you may be invited to join the project as a committer. This happens after someone at the PMC proposes your name and a vote is started. After the majority of PMC members cast their votes and there are no vetoes, an invitation is sent.
There are no recipes to join the project. Just subscribe to the mailing list and participate with opinions, patches and reviews.
Castle development discussion takes place on the devel mailing list. You do not need to subscribe to the list just to submit a patch or two, but if you want to be involved with Castle development on a regular basis, you should probably subscribe. It can be high-traffic, but threading tends to be fairly disciplined, so you can ignore conversations you are not interested in.