moodle/lang/en/help/groupenrolmentkey.html

<h1>Group Enrolment Key</h1>

<p>Enrolment keys allow users to enrol themselves into a course, usually there is one key for the whole course defined in the course settings.  However, if you define a *group* enrolment key then not only will entering that key let the user into the course, but it will also automatically make them a member of this group.</p>