How to email individual content to email field?
If you're looking for existing module to base a solution on. The Print module has a Send by email submodule that supplies a form (/printmail/node/NID) that you can tweak for sending out emails and capturing the supplied emails.

/**
 * Implements hook_form_print_mail_form_alter().
 */
function MYMODULE_form_print_mail_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  // Add a custom submission handler for your functionality.
  $form['#submit'][] = 'MYMODULE_form_print_mail_form_submit';
}

function MYMODULE_form_print_mail_form_submit($form, &amp;$form_state) {
  // @TODO: process $form_state['values']['txt_to']['addrs'] for email addresses to store.
  // Use $form_state['values']['path'] to the load relevant node &amp; save data to its fields.
}