As a User
There are many ways in which you can contribute to the cause. The best, and probably the easiest way is by using the software produced by the projects (i.e. Ja.NET SE at this point). Download the software, install it, try it, report any bugs you find, make feature requests and participate in or start discussions on the direction the software is taking. We very much value the input on what we are doing from others, and are very open to feedback.
As a Developer
You can also become involved as a developer. You can get involved with the Ja.NET SE project by providing patches for bugs, by submitting code for new features, by participating in forums or by improving the documentation.
You can also start your own Java for .NET project. It could be something as simple as taking an existing OSS Java library and adapting it for .NET using Ja.NET SE and then providing it as a download in the download center. Or it could be as large as starting a Ja.NET ME project focused on delivering a Java ME for .NET that runs on Windows Mobile. It's your choice.
If you're not sure on how or what you can do to contribute, then feel free to ask. You can bring it up on one of the forums or just use the "I Want to Contribute" portlet to send us a message.