If you're a participant, and you haven't been asked to be an editor, then you're an author. (Editors are authors too.) So, what can authors do? Almost everything. Authors can start new articles, edit existing articles, talk things over on the talk page, and much else. Editors can do all that, too. Editors aren't distant overseers looking down from the Olympian heights. To edit, editors must work shoulder-to-shoulder with authors and other editors. So editors are authors too.
Editors, who are experts in their fields, do have two special functions that authors do not have, however. First, they may make decisions, where decisions need to be made, about how an article should read. (This does not mean that editors must approve every change an author makes, which they certainly don't do.) Second, they may approve specific versions of articles. If you're an editor, and you want to know how to get started as an editor, please see The Editor Role. Some editors are members of one of the governing bodies known as the Editorial Council.
Where do constables fit in? Well, like their namesakes, constables are friendly, hard-working folks who make sure the community runs smoothly. If you break a rule, a constable might gently tap you on the shoulder and explain what's wrong. Constables are expected to be mature and kind and to make decisions solely about behavior, not about content, which is the domain of editors. See the Constabulary homepage for more. If you need to "call a constable," send an e-mail to email@example.com
Finally, workgroups are collections of editors and authors who are interested in a particular discipline, or in some function of the project, such as copyediting or recruitment. We know that our list of disciplines needs work, by the way; the Editorial Council will be expanding the list soon. In the coming months, we're going to be expanding our governance apparatus, setting up many more workgroups as well as a general project representative council. You, as a project member, may be asked to serve.