Our Slack channels are not generally open to just anyone; we typically
require people to have actually contributed code to the projects in Chromium before we can send them invites. Once you've contributed something, we'll happily send you an invite. If you have submitted something and your name is just missing from
the CONTRIBUTORS file, we should fix that. In the meantime, I encourage you to just use the public graphics- and ANGLE-related mailing lists.