Occasionally Teachers may prefer to allow students to self-enroll in their course in order to more easily facilitate course joins. This can be especially true in Open and Training spaces that do not have enrollments fed to them by Banner and other upstream systems. This process is also detailed in the Canvas Community here:  How do I enable course self-enrollment with a join... - Instructure Community (canvaslms.com) 

Define a URL in your course for student self-enroll:

• navigate to your course in Canvas and click "Settings"
• scroll down to the bottom and click "More Settings" just under Description
• check the box next to "Let students self-enroll by sharing with them a secret URL"
• click "Update Course Details"
• the page will automatically refresh...scroll down after it does
• your custom URL is shown and starts with "https://canvas.illinois.edu/enroll/"
• copy the URL and share as needed for student self-enroll

Things to consider:

• students will still have to authenticate with the University enterprise authentication (i.e., NetID and password) at minimum
• guest users will be able to take advantage of this, but will have to be setup in the Canvas environment first
• there is no federation beyond the Urbana campus (i.e., no BTAA, Educause, etc.)