This part is easy! All you have to do is visit https://runcode.ninja and click on the Register link at the top of the page.
Slack Chat Invitations
When you receive your registration email, you should also receive an invitation to join our Slack chat channel at runcodeslack.slack.com. Slack chat is available through any HTML5-compliant web browser, through mobile apps for iOS and Android, and via Desktop Apps for Windows, Linux, and MacOS. We strongly encourage you to join us on Slack - there you’ll get feedback on your submission results, help from the admins, and you’ll also be able to connect with your fellow players.
Invite Your Friends
Each account comes with the ability to send invitations to join RunCode. If you do not have any invitations, you can ask an admin in slack to provide you with some. You can access this feature through your RunCode profile or there is also a link at the top right of the navigation panel on the website (you must be logged in).