Hi Stefan,
This functionality only works in case an OnPremise (SCIM) User Provider is configured for your account.
If you use the default configuration - for SAP ID Service - this is not working due to security reasons (due to Data Privacy Regulations, SAP ID Service cannot give you any information about other registered users).
It is possible if you have your own tenant in SAP Cloud Identity Service. In this case - the APIs part of the HCP SDK are not yet able to do this, but SCI has it's own Rest Based APIs that you can use to browse the users
Regards, Vladimir