The Shared Albums feature in Google Photos is getting smarter and more useful in the near future, when the rollout of the app's newest versions will be complete on Android and iOS.
In Google Photos 1.10 for iOS and 1.20 for Android, people who join an album can now add comments to photos and even to the album itself. There's still no way to Like or +1 anything, though.
Additionally, the service now makes it easier for you to add your own photos to an album someone else has shared with you. That's because you now get smart suggestions for which photos to add to such a shared album - no need to go digging through your collection anymore. If the suggestions are spot on, then you can add your photos from a certain event to a shared album with just one tap.
This probably works by pairing the time and place your photos were taken with pics already in the shared album. If the app notices you've snapped some images in the same place or at the same time, it will recommend them to you for uploading to that album.