There are many ways to contribute to JBoss Portal. Any form of contribution is welcome but here are some example domains.
Letting us know on the forum of any bug you met, is the first way to contribute. After discussion and if it happens that it is really a bug, submitting it to http://wiki.jboss.org/wiki/Wiki.jsp?page=PortalJIRA will make sure that it doesn't get lost.
Updating the Wiki with some tips and tricks you are using helps the others. You can also provide patch on the official documentation written using the docbook format (it is available in the SVN repository).
You are good are drawing schema's? A good architecture overview schema would worth a thousand words, please if you do so make sure you are using a tool that can be used by others (OpenSource, available on multiple platform).
A picture of the interceptors would help too. A picture of dependencies between modules as well. We can help you.
Of course code contribution is also welcome, if you fixed something or improve something, please consider submitting it. If your patches don't break the code you will get commit access to modify the source code yourself.
Everything you submit is time that other developers save and can spend on working on new features of JBoss Portal instead of fixing what others have privately fixed or enhance.
If that's your domain of expertise, we would love to receive your mockups of a better GUI or better usability of the portal and embedded portlets. You can also contributes themes and layouts.
And so on...