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You may want to add an enrolment key for guests, this can be the same, or different to one for students.
From within your course, go to the Settings block, if you are within an activity you may need to expand the Course administration sub-menu. Under Course administration click on Edit settings.
This list contains a lot of course-wide settings but it also includes a group of options for Guest access, scroll down to find these:
To add a password for guests, set it in the Password field.
Click Save changes (at the bottom of the page)
Once the guest access has been set, you can then advise your guest to go to moodle.ucl.ac.uk and click on the 'log in as a guest' link, which appears within the login box on the Moodle site home page. Once they have accepted the site terms and conditions, they will be able to find the course.