Add one invite. Check "Attended (going)" if they came to the event so they receive the post-event thank-you.
Paste CSV or tab-separated data. First column must be email. Optional: second = first name, third = last name. (Eventbrite export: use Order Email, First Name, Last Name columns.)
Imported rows will be added as invites. Check "Mark as attended (going)" to send them the post-event "Thank you (attended)" letter.