Anyone may propose a new Community Group. You must log in to propose a group (or request a W3C account). Once you are logged in, you will provide the name of the group, a short description (no more than a small number of paragraphs), and a short name. The shortname is used in various places (such as automatically generated URIs or mailing list names) to refer to this group. The shortname must not have spaces and any characters must be lowercase.
Once five or more people (including the person who proposed the group) indicate their support for the group, W3C creates the infrastructure for the group (mailing lists, blog, wiki, etc.) and announces the new group. At that point people may join the group and get started.
You may also wish to consult the lists of existing and proposed groups.