in your w3c account go to “Connected accounts”, make sure your github account is linked otherwise it will not show up on the CSSWG participant list and a bot will flag your pull requests
Get yourself invited to the w3c GitHub organization, and accept the invite. You can make your membership public if you want. This (plus the account linking above) should mean you’re automatically added to https://github.com/orgs/w3c/teams/w3c-group-32061-members (where you can check whether you're in the groups). The invite should be doable by any W3C staff. (TODO: Is this step still needed?)