To contribute to JDK Mission Control, you (or the company you work for) need to have signed an OCA, like for any other OpenJDK-project. If you already have an OpenJDK username, you can associate your GitHub account with it.
Just after we open sourced JMC, I created a temporary mirror on GitHub to experiment with working with JMC at GitHub. That mirror is now closed for business. Please use the official OpenJDK one from now on:
If you forked or stared the old repo, please feel free to fork and/or star the new one!