The code is on GitHub and we will be letting people in batches to MuditaOS private repository. We're currently finishing working on documentation, CLA, etc. so that anybody who joins the repo will be able to start contributing as soon as possible.
This is happening on top of our day-to-day development activities so bear with us - we'll send out emails with next steps to all people interested soon.
Thanks! I work at Mudita so I'll let you know more :) After singning up for the Developer Preview we will invite people in batches to the closed GitHub repo. We're still tinkering the docs and very actively developing the OS. When it comes to the license, we're aiming at GPL 3.