b2 is a build management tool specifically designed for software that is based on the Eclipse plug-in technology.
To be more precisely, b2 is a solution to build, test and deliver OSGi bundles, Eclipse plug-ins, Eclipse features, Eclipse applications, Eclipse products, Rich Client Platform (RCP) applications and the corresponding p2 update sites in a standardized way and with the least possible effort.
The simplest way of contributing is probably to report issues. You can do so using the Issue Tracker.
If you want to contribute your code or just want to share it with others, you can create a fork of the official repository at any time, for which you will have full access so that your local changesets can be pushed to it.
Once your code is ready and accepted (see code style section below), it is then easy for the project owners to pull your changesets into the official repository - all you have to do is to create a pull request.
For general information see Contributing to Open Source on GitHub.