There are several ways you can help the project.
We have found that not many people are aware of Mutation Testing. You can help by simply asking your colleagues or friends about their opinion on mutation testing.
Try using Mull on one of your projects, or on one of the many open source projects. Mull uses JIT for performance and flexibility, which is inherently complex and easy to get wrong. We covered some cases, but certainly not all of them. The project would benefit from more samples.
You might get confused by the documentation, or by our design decisions. Some of the decisions are done on purpose, some of them are accidental. If something is not clear, confusing, or can be improved - please open an issue and raise your concern.
If you gave it a try, then you probably hit some bug or crash. Please, file a report, we will take care of it eventually.
There is a non-exhaustive list of planned features. Feel free to implement any of them, or come up with some features you think would be beneficial for the project.
Here is the starting point: Hacking on Mull.