Subscribe to Shared Album from iCloud for PC
You cannot subscribe to a shared album without a Mac or an iOS device.

See this message from Apple when trying to subscribe to a shared photo album:


  iCloud Photo Sharing is not available for this device
  
  To subscribe to (sender's name)’s “Sample Photo Album Name” shared
  album on your iPhone, iPad, iPod touch, or Mac, open your invitation
  in the Mail app, and click the Subscribe button in the message.
  
  
  
  To subscribe you need to be signed in to iCloud on:
  
  
  an iPhone, iPad, or iPod touch with iOS 6 or later, or
  a Mac with macOS 10.8.2 or later and iPhoto 9.4 or Aperture 3.4 or later