Helping with MoinMoin development
develop new extensions (macros, actions, processors, parsers)
run the development version and help testing the new features
test MoinMoin on a certain platform, or with certain Python versions
help with the translation efforts, especially adding new languages
help developping new themes, CSS and icons
In any case, you should subscribe to the MoinMoin Developer List. More information on MoinMoin source code and other developer issues can be found in the MoinMoin main wiki, on MoinDev and its subpages.