For each W3C group there is:
Those resources are linked from any call for participation in the group and the list of groups.
A Member's Advisory Committee Representative joins any group (that is more than a mailing list) on behalf of that Member. If you wish to join a group, please first contact your Advisory Committee Representative.
Here are the general instructions for Advisory Committee Representatives (assuming they have a W3C account; if not please complete the Member Access Request Form):