One way you can track usernames of players is to mirror the Player collection on Registration, so that you can perform queries in Cloud Code on fields such as "userName".
Sorry for reopen this thread, but there is still one detail i`m in doubt.
Every request must be done with authenticated users, right?
So how can an unregistered user check if an username is already in use?
Consider I will use RegistrationRequest and my game username will be an email.
There's no way to check that before send the RegistrationRequest, right?
Thanks in advance.