You can help Cricket grow and continue to be useful to people in lots of ways. Perhaps the best way to get started is to help provide support to new users on the mailing lists.
When you are ready to start hacking Cricket itself, another good place to start is documentation. Take a look at the list of bugs on SourceForge. Experiment with the filters until you get a list of the documentation bugs. Choose one, assign it to yourself, and then type away. If you've made a change locally that you'd like to share, talk about it some on the Cricket developers mailing list, and when people agree it's a good idea, check it in.
Here are documents that may be of use to developers.
Here are some links to resources on the net that are useful to Cricket developers: