The Xfce GitHub organization is a read-only mirror of most of the repositories of the Xfce Desktop Environment. You can use it to view the source code of the project, fork it to do whatever you want. However because this is a mirror, it is not used for contributing to the project.
Submitting bugs and patches
Xfce has a separate tool for submitting bugs and patches. You can read all about this in the bug reporting section. The Xfce developers do not monitor any issues or pull requests filed against the GitHub mirrors.
You can however use your fork on GitHub to provide complex patches and keep them in sync with the upstream repository in a special branch.
Not everything is on GitHub
Modules are created and updated on-the-go when a change gets pushed to a repository. Some repositories are also excluded from the mirror (user repositories from Xfce developers and the archives). For all repositories visit git.xfce.org.