Those you have shared projects with can rate the files, but first you need to set the share permission for them to do so.
In the permissions overlay (link) you will need to activate the 'Collaborate' option. Once done this, your collaborators (or the ones that you have given this permission to) will be able to rate your files, even when they are not users. You can let everyone you share with add their ratings, and an average of the rating will appear below the image when you activate the ratings viewing:
Only you can see ratings from other users.
This option is extremely useful when making image selections with people you collaborate with, editing process or getting feedback from a client, curator, editor or another artists.
To learn more about rating files, click here.