JChemPaint is heavily based on the Chemical Development Kit (CDK). JChemPaint (licensed GPL) only contains the GUI wrapper (menu's, toolbars, etc) around the chemical functionality from the CDK (licensed LGPL).
JChemPaint and CDK are mantained as SourceForge projects under http://www.sourceforge.net/projects/JChemPaint, and http://www.sf.net/projects/cdk/. SourceForge gives us bug tracking, mailing lists, support manager and last not least CVS access for source code development.
JChemPaint and CDK are maintained as SourceForge projects under http://www.sf.net/projects/jchempaint/, and http://www.sf.net/projects/cdk/.
There is more than one way to participate:
Each of these possibilities is equally deserving, although some people rank coding highest.
If you want to write documentation or start coding, please check the SourceForge site documentation on how to access the CVS. If you actively want to contribute, please get a SourceForge account, contact the project admin so that he can add you to the developers list. The next step would be to checkout the source code.