Adding message to email when using addEditor()

As documented, file.addEditor() has only one argument: an email address (or a User object). So an email message cannot be added via that method.

However, you can accomplish this using the Advanced Drive Service. First you'll need to enable the Advanced Drive Service.

Then use something like this (reference):

Drive.Permissions.insert(
  {
    'role': 'writer,
    'type': 'user',
    'value': email // the email address you wish to share with
  },
  documentId, // id of the file you want to share
  {
    'sendNotificationEmails': 'true',
    'emailMessage': message // The message you want to send, as a plain text string
  });