Advice and answers from the Leanpub Team

This is happening because your co-author does not already have a Leanpub account associated with their email address. If you ask them to first sign up here https://leanpub.com/sign_up then you will be able to add them as a co-author.

Now, we should definitely display a message when this happens and explain this on that page, sorry for the lack of clarity.

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