Welcome! Thanks for your interest in contributing to Chassis. You're almost in the right place. More information on how to contribute to this and all other jQuery Foundation projects is over at contribute.jquery.org. You'll definitely want to take a look at the articles on contributing code.
You may also want to take a look at our commit & pull request guide and style guides for instructions on how to maintain your fork and submit your code. Before we can merge any pull request, we'll also need you to sign our contributor license agreement.
You can find us on IRC, specifically in #css-chassis should you have any questions. If you've never contributed to open source before, we've put together a short guide with tips, tricks, and ideas on getting started.
Our weekly meetings on Tuesday at 2pm ET on IRC in #jquery-meeting. These meetings are public and all are welcome to attend and participate. If you can't make the meeting, you can find the internal notes for each meeting on the jQuery Meeting site.