Hello, I am facing the same problem for some time now.
In fact I had two Moodle instances where I was testing my Web Services Client and in one of the two core_get_users was working and on the other it didnt work,
The one, that worked, is version Moodle2.7.2+ (Build: 20140911)
Anyway, following your suggestion I gave the permissions you mentioned to my other instance (Moodle2.7.2+ (Build: 20141002)) but I am still getting an empty response.
The only way to make this work is to give Administrator role to my webservice user. But even when the webservice is administrator, when I make a call I dont receive all the information specified in the API Documentation, I get less.
Is this normal? Could you please let me know if you managed to fix it and how?
Many Thanks
Christos