Hi Joe,
No it is not an entirely different registration site, it is all built within the same event in Cvent. If you add the modify registration widget onto the confirmation page in your event, attendees can log back in and modify their registration and add on sessions in the registration process and /or if you are using the Attendee Hub for your event, attendees can also log into Attendee Hub add on sessions within the app or Attendee Hub website. Reference the section "Allowing Invitees to Modify Their Own Registrations" in this article Modifying Registrations for more information.
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Julia Plymack
Senior Customer Success Manager
Cvent
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