If you received the following error message while trying to add Registrant via the "Add Registrant to Webinar button on Leads or Conact: Error message:List has no rows for assignment to SObject Error is in expression '{!sendWebinarId}' in page crmg_gtm:addregistrantstowebinar: (CRMG_GTM), try this:

Go to GoToWebinar Users > Click on All > Review your list of Webinar Accounts and make sure that there are no duplicate credentials. 

In other words, you can only have a GoToWebinar Account listed one time here. Your Salesforce org can have dozens of GoToWebinar Accounts, but no duplication of exact credentials. 

Delete or make "Inactive" one of the duplicate credentials.