Hi Amy,
In order for invitees to be able to RSVP yes or no to attending the event via the save the date email, you will have to open your event up for registration first (make sure that registration is in the active status.)
You can use the RSVP Yes data tag within your email for people to rsvp yes and be brought into the registration process to register for the event and the RSVP no data tag for those to rsvp no that they cannot attend the event. Data Tag Cheat Sheet
Once someone registers or declines, they will either be in the accepted status or the declined status in the event, so they not receive the invitation email that is set to those "undecided invitees" who have not responded either way. Make sure that the send settings of the invitation email are set to go to "Undecided Invitees."
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Julia Plymack
Senior Customer Success Manager
Cvent
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