A course enrolment (or enrollment) key is one method of restricting Self enrolment to a smaller group. The default setting is not to set a key and allow anyone to enrol themselves into the course.
An enrolment key is often used as a simple method of having someone else beside the teacher determine if a particular student can self enrol in the course. The idea is that one or more people will supply the course key to authorized people. Sometimes this distrubution is private email, snail mail, on the phone or even verbally in a face to face meetings such as a class or counselor's office.
Setting an enrolment key within a course
- In the course settings block, click Users>Enrolment methods
- Make sure self enrolment is enabled (has its eye open) and then click the edit icon on the right:
- Add your enrolment key in the box provided. (Click Unmask to see what you are typing.)
- Click Save changes
Setting a group enrolment key
- It is possible also to enrol students directly into groups by use of a group enrolment key.
- To do this, you first need to add the course enrolment key as in the section above.
- Change the setting Use group enrolment keys to Yes
- Save the changes and then in the Course administration settings, click Groups>Create groups
- Add an enrolment key to the group. This will be the key they type in to access the course. (The course enrolment key is simply there to keep others out)
- See also this screencast: Group enrolment keys in Moodle 2.0 
Making an enrolment key required in every course
- By default, enrolment keys are not set. However, if the administrator wants to ensure all teachers set enrolment keys then this can be forced in Settings>Site Administration>Plugins>Enrolments>Self enrolment This means that self enrolment settings will not "stick" until the teacher adds an enrolment key. NOTE: It does NOT provide a site wide enrolment key.
Using the password policy for enrolment keys
- To ensure that teachers use secure keys that follow your Moodle's password policy, check the box in Settings>Site Administration>Plugins>Enrolments>Self enrolment
Giving users a hint of the enrolment key
- By default, for security reasons, Moodle will not give any clues as to what the enrolment key is. Checking the box in Settings>Site Administration>Plugins>Enrolments>Self enrolment will provide the first letter of the enrolment key if needed.
Tips and Tricks
- If the enrolment key for a course "gets out" and unwanted people self enrol:
- Changing or placing a key does not impact currently enrolled students, nor does it impact students who may also be enrolled by an enrolment plugin.
- Guests may (optionally) be required to supply the enrolment key (as specified on the Course Settings page). They will be required to supply the code every time they enter the course.
- The "No" or "Date range" options in the "Course available" setting effectively disables the enrolment key feature, even if it is set.