Following the release of App Preview Messaging for Google Allo and Google Duo, support for the feature has now been added to Google Photos. If you don’t recall what App Preview Messaging is exactly, it is a function that allows those who don’t have the app installed to get a notification on their Android phone. In this case, they can then action that notification by viewing the photo or, optionally, installing the Google Photos app.
The feature works when someone shares a photo with you from Photos and sends that share notification to your mobile number. This will trigger the notification which will have the name of the sender. If you don’t know the sender, you can block them or you can open the notification to see the content. From there, you are taken to the Photos website where you can install the app on your phone.