title: Can external user access spreadsheet on SharePoint by using API?
body: <p>A bussiness partner had shared a spreadsheet on SharePoint to me.
  I can access and edit it by using my email &amp; Verification Code in firefox.
  now I want to access that sheet by using API.
  however, I can find the API for internal user, but not for external users.
  So, my question is there any API for the external users to access SharePoint?</p>