Under "Site Settings" --> "Manage Access" in any CourseWorks site, there is an option called "Global Access". If you select "Allow anyone to join the site with valid login id" then the site will show up in all CourseWorks users' list of "joinable" sites (which can be found under the "Membership" tab in the "My Courses" area of CourseWorks).
This feature might be useful with a project site (e.g. for use with a group of administrators or a study group for students) where the convener wants to allow anyone with a UNI to opt in to the course site. In this case, the convener would not have to manage each request for access, rather members could simply add themselves.
There are special access options for "self-join" attendees in the permissions settings for most of the tools within CourseWorks. This allows the convener to tweak access for this group of members (usually restricting them from accessing certain parts of the site).
Self-join access is different than "shopping period" access, the settings for which are also on the "Manage Access" page. The shopping period settings allow faculty to set (or override department settings) about who can see their course site. Self-join takes this a step farther by allowing members to actually join the site.