Google has added a new feature to Google Photos that make it easy to start a private photo and message thread with one or more contacts.
When sharing a photo, the new choice lets you start a new thread or add to an existing one, making it easier to group a series of photos shared with one or more specific people.
All participants can also add comments and likes, and download full-res photos.
As a side note, YouTube recently removed its direct messaging option. But never fear Google seems to love adding and removing messaging apps and system.