Good evening Melissa and welcome to the Events Calendar Support forum!
Thank you for reaching out to us.
I would love to help you with this topic.
The simplest solution to this would be to use a WP user switching plugin (shop as a customer is just one of many of these).
That would allow the client to complete the normal order process for the attendee, thus adding the attendee to the attendee list.
As a side note, we are also working on an extension that would allow to edit attendee info and add it manually. We will let you know as soon as it is out.
Best regards,
Geoff B.