Git, GitHub and GitLab
Git is the version control system adopted within the NNPDF Collaboration. Among other things, Git allows multiple people to edit code simultaneously; it allows users to track changes to the code, i.e. a user can see who edited what and when; and, it allows a user to view any past version of the code. The latter two points are particularly useful for running tests should a bug be introduced to the code, for example.
Using Git can be slightly confusing at first, but by mastering a few basic commands you will be able to do most of the tasks that you need to do day-to-day. Git also has the advantage that at the moment it is probably the most popular version control system out there, so any time you in invest in learning Git will most likely be useful for projects outside of NNPDF. Many online tutorials and guides exist for learning Git, but here are two that I have used before: a quick guide and a more in depth tutorial. The official documentation might be useful as well.
GitHub development workflow
GitHub provides the following workflow:
Users can create Projects and Milestones for each project.
For each project users can open issues, which can be used to request bug fixes, new features, new documentation, or simply to facilitate a general discussion. The user can then assign one or more people to help deal with the issue. Issues should be opened in the relevant repository. For example, for something that is related to validphys, one should open an issue in nnpdf, while for something that is related to data implementation, one should open an issue in buildmaster. Example issues can be found here.
When it is clear how an issue should be dealt with, a branch can be opened where a user can implement the requested feature.
Once a feature is ready to be considered for merging into the master version of the code, a pull request (PR) can be opened. At least two code reviewers must then be assigned, after which the code will be reviewed and discussed. The modification will then be accepted or rejected. Further general information on PRs can found here.