Hi,
If they checked off the box for "i am registering on behalf of this person" and entered in their email address as the administrator email address in the section highlighted below and not the attendee email address section above it, then they will be able to register themselves for the event without issue. However, if they entered in their own email address for the attendee email address and are then trying to register for the event themselves with the same email address, then they will not be able to do so. Two people cannot be registered for the event with the same email address. If that is the case, I would recommend that the email address of the first person who was registered is swapped out to be their own vs. the other persons.

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Julia Plymack
Senior Customer Success Manager
Cvent
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