Another alternative: "Self registration with no email confirmation" https://moodle.org/mod/forum/discuss.php?d=354562#p1455304. Tested just the other day against Moodle 3.5. Simple and sweet.
Yes, this is an invitation to all sorts of spammers, if the server faces the Internet. My application was inside an "Internet-less" lab.
Security and privacy
Fast registration possible?
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