In your event's left navigation, go to Guest responses > All Responses. You'll see the number of guests who have responded, along with the full response list at the bottom of the screen.
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Locate the response you want to update and click on it. The guest's submitted information will appear on screen. Click Edit to make changes.
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Make your changes and click Save. The guest's response will be updated immediately.
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In Guest responses > All Responses, locate the group you want to update and click Add Guest to Group.
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Enter the new guest's details and save your changes.
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