To add a collaborator, go to the book's folder in My Drive.
To the right of the folder name at the top of the page, you will see a little circular "people" icon.
Click on that and use the email address of the person you want to invite as a collaborator.
Please note that this just makes them a collaborator on the Google Doc, it doesn't make a co-author of your Leanpub book.
Here are some instructions for adding a co-author to your Leanpub book: http://help.leanpub.com/author-help/general/i-have-co-authors-do-you-support-having-multiple-authors-for-a-book.