by Nitin Jain.
For me it didn't work even after providing moodle/user:viewalldetails permission..
then got some reference from: https://tracker.moodle.org/browse/MDL-42639 and added one more permission to my webservice role "moodle/site:viewuseridentity" and it worked..
then got some reference from: https://tracker.moodle.org/browse/MDL-42639 and added one more permission to my webservice role "moodle/site:viewuseridentity" and it worked..