One of the frequent support requests I receive for the RSVP Pro plugin is questions surrounding the attendee import functionality. When the import process was created eight(!) years ago. You had to have a specific file layout in your spreadsheet, and it was rather fragile. The import process was way overdue for an overhaul.
With the release of version 3.8.7 the import process is much easier to use. Here is a video showing the new workflow when importing attendees.
With the new process as long as you have your attendee information in a CSV or Excel file, you can import it into your event. The only required field is either a first OR last name. You can read more details in the updated documentation.
I hope it improves your the workflow. If you have any feedback, please contact me. I love to hear from my users.