Here are the steps for adding a co-author to a Leanpub book:
1. Ask each co-author to sign up for a Leanpub account, if they do not already have one. It is free to sign up. Here is a link to the sign up page: https://leanpub.com/sign_up.
2. Ask each co-author to provide you with their Leanpub username.
3. After you have created your book, add each co-author here:
...making sure to replace YOUR_BOOK with your book's unique web address.
On this page, you can use each co-author's Leanpub username to invite them to join your book.
4. Each co-author then needs to accept your invitation. They can accept the invitation here: https://leanpub.com/user_dashboard/invitations.
(If you are looking for instructions on how to remove a co-author, please read this article.)