If you have not yet completed the purchase of your book, you can add up to 5 additional characters to go along with your primary character.
When you get to the section of selecting your characters, simply add them by clicking on the Add Another Character button, enter their information and upload a photo for them.
Once a book has been generated however, a new characters cannot be added.
If your book is currently Paused, you do have the option to transfer your purchase to a new book and add your new character. Your prepaid printing and shipping will transfer to the new version, so you will not need to purchase the physical book again.