One of the best things about open source in general, and the modern "social collaboration" workflow in particular, is that participation isn't restricted to a core development team. Anyone can fork a project at any time, make some changes to it, and submit those changes back for review and incorporation into the software, potentially sparking a long and productive collaboration.
Because the Foundation uses public GitHub repositories for all source code management, their process is what's used by the Foundation to support this workflow.
If you have an interest in participating in a project but you're not sure where to start, some project teams will label issues with
help wanted or
good first issue tags; ideas to get involved are also socialized in the foundation's "This Week at FINOS" weekly email (published via the Community mailing list; to the email@example.com). These issues and calls for help are a great way to get started on contributing to a project, as they've been pre-vetted by the project team and generally won't be overly complex or time consuming to implement.
These are also highly valued contributions, and project teams typically appreciate this kind of input just as much as code-level contributions. Most projects host an issue tracker on their GitHub repository, and if not will provide instructions on where issues should be raised instead.