We discovered that "Event Attendee" group names are based on the very first title that was saved for the event. ( Read more about that in the Knowledgebase ) If you change the name of the event, you have to either remember the previous name to find the group (not likely), or contact support (not ideal).  I'd like to suggest that Event Attendee group names either change as the event name changes, or is matched with the Event ID the way that "Advocacy Participants" can be match with alert ID.