The event is set up to auto-create users and assign them to the Registered group. When the user registers for the event, they are prompted to enter an email and password, and get no error message.
They get registered to the event fine, but looking at the Joomla user table... their user account is not created.
I verified that the event does have auto-creation of the user set to "required".
I hope you have better luck with this if you are accepting payments and allowing them to change their registrations. This caused too many problems for me so I stopped allowing user creation for edits. Example: They register and pay as earlybird, then log in after earlybird expires to add somebody at the regular rate, and it will show a balance due for the original registrant. Or registrations get orphaned/damaged during the user creation, or users get created but it does not put them in the assigned group from time to time so those users can't log in.... Stuff like that. 50 percent of my problems stopped when I stopped using those features.