Luminate Online automatically creates a group for Event attendees. To find it, follow these steps:

  1. Go to Constituent360 > Groups
  2. Search using the name of the desired Event
  3. Look for the group that corresponds with the Event name and date of the event. It will look similar to this: "TestEventName 08/17/2017 12:00 PM" 

You can use this group as a target group for an email delivery.