If you are interested in working with us, let us know using our mailing list! Then we recommend reading the How to Contribute section of our Developer Guide.
We are interested in any contribution. There are many ways how you can contribute, not just code:
Additional ideas can be found at our wiki.
To get acquainted with SfePy, you can start by reading the Tutorial and Primer sections of the documentation and trying out the examples that come with the sources. Your first contribution could be pinpointing anything that is not clear in the docs.