NodePen has recently changed its license and is fully open to contributions.
In the near-future, though, I have a number of known features to implement and architectural decisions to make. This will limit the usefulness of outside contributions. If you are eager to contribute, I've begun to add a backlog of "needs help" issues to the repo. Feel free to pick them up or reach out in repo discussions for details.
After the 2.0 release, there will likely be better and smaller pieces to pick up.