Set an Enrolment Key (and an Enrolment Keyholder)
The enrolment key is a one-time used password which gives any logged in Moodle user (think: potential student) access to your course. They are useful because they can be memorable, given out to students but also prevent others from accessing your course materials.
From within your course click the Settings navigation tab, if you are within an activity you may need to expand the Course administration sub-menu. From within Course administration expand Users and then expand Enrolment methods. It's a bit hidden away but click on Self enrolment (Student). This will load a page of options, including where to set the Enrolment key. If no key is set, you may enter any key phrase or term into this area.
Make the key easy for someone else to copy/write down but don't set it to the same as the course or module title - as these are ineffective. You may click the Unmask option to view your key in plain-text.
There are are few other options in here too, such as student enrolment duration, start/end dates, maximum number of enrolled users and the course welcome message - which is emailed to all new students in your course (and can be an effective introductory touch)
Click Save changes to finish.
Students may now browse to your course and enrol with that key.
Set the Enrolment Keyholder
The Enrolment keyholder is the person who is listed as a contact point if any user struggles to enrol in the course, or requests to know the key. By default, Moodle tries to guess the best member of staff to show as a contact, but a keyholder can be explicitly stated.
TBC - not sure how this is done yet!