Hi Chris,
Are you importing events/attendees from another calendar plugin or from one site using The Events Calendar to another?
If you’re migrating data from one site to another and both use the plugin, then I would suggest exporting the database from first site over to the next.
If you’re migrating data from another system altogether, then it may not be possible to move that data cleanly without some more advanced database work. In this case, it depends on how the other system handles attendees and then mapping them to The Events Calendar’s attendees.
Cheers,
Geoff