At I/O 2017, Google announced a number of updates that were coming to Photos later this year. Until then, the Android app appears to be testing a new sharing interface that is better optimized for selecting multiple images at a time.
Spotted by Droid-Life, tapping the share button still slides up the same sheet as before. However, there is now a carousel of images up top, with a check mark in the top left corner denoting selected pictures.
From here, users can easily swipe back and forth through this timeline and select other images to share. One of the biggest improvements is how users can now see a full preview of the image instead of having it covered by the share menu.
Additionally, this carousel doubles as a convenient way to double-check that you have the intended photos selected before sharing.
More recent updates to Google Photos include removing the ability to back up only when charging and a new Archive feature for hiding pictures from the main feed. The built-in Assistant feature can also suggest what photos to archive and is geared towards hiding documents and receipts.