Navigate to an Event and click on the RSVP tab. Here you will find a tab on the left entitled ‘Calendar Invite’. Make sure you have that clicked. On the right, you will see your calendar pallet.

Draft your description. Make sure you keep the RSVP link in there as shown. Did you notice the auto save at the bottom?

After you draft your email, return to your Campaign and when guests respond affirmatively to your event, they will automatically received the email you customized.