In the first place, GitHub is a webserver for Open Source projects. It stores, visualises and helps to manage projects that develop some kind of computer code. Of course everything on GitHub is under git version control.
To participate actively in the code development in WP6 you will need a GitHub account, which is completely free (no adds, no personal data trade).
Then, you should join the CASCADE WP6 team at github.com/cascade-wp6 which gives you access to the private repositories. Now, you also can
GitHub provides a couple of features for project management(Milestones and issue tracking) and documentation (Wikis and Version history).
GitHubs has a very supportive research policy and provides non-commercial research and educational projects with free-of-charge private repositories. As an individual researcher you can apply as well for your own projects.
Thus all the useful features of GitHub are available to our ongoing projects within the WP6 team. And we can liberally choose between private or public development.