Hook to add new field to node form
I've never used it myself -- but see Inline Entity Form which should allow you to reference an existing author, or implement that modules form controller class to show an author and email field to create the new author as needed.


  Provides a widget for inline management (creation, modification,
  removal) of referenced entities. The primary use case is the parent ->
  children one (product display -> products, order -> line items, etc.),
  where the child entities are never managed outside the parent form.
  Existing entities can also be referenced.


From the README

Integrating with Inline Entity Form
-----------------------------------
An entity type can add support for this module by declaring the
inline entity form controller class in its entity info:

$entity_info['commerce_line_item']['inline entity form'] = array(
  'controller' => 'CommerceLineItemInlineEntityFormController',
);

The controller needs to extend EntityInlineEntityFormController and at least
override entityForm() to provide a function entity form.