develop new extensions like macros, actions and parsers (MoinMoinExtensions)
run the development version and help testing the new features or test MoinMoin on a certain platform, or with certain Python versions (MoinMoinTesting)
help with the translation efforts, especially adding new languages (MoinDev/Translation)
help developping new themes, styles and icons (MoinDev/ThemeDevelopment)
If you want to help you should look on existing bugs at MoinMoinBugs and look what user want at FeatureRequests.