GitHub is home to over 28 million developers working together to host and review code, manage projects, and build software together.
If you're willing to help, but need ideas, here are some suggestions:
Pick up an issue (how to write issues), and give your opinion or contribute code.
Functional automated testing is done with the great Robolectric framework. If you feel like rising up the test coverage, please go ahead ;-) .
Improve documentation (either the Javadoc or the wiki).