In Reporting Services you can manage permissions for existing users (e.g. windows users in your Active Directory domain or workgroup), but you cannot create or delete users. Assuming that you are using Power BI Report Server or Reporting Services in native mode, they work with Windows Authentication and also support third party custom authentication.
Here is a good article about granting permissions (Note: if you are using Power BI Report Server, the "Native mode report server" is the topic you want to look at): https://docs.microsoft.com/en-us/sql/reporting-services/security/grant-user-access-to-a-report-serve...
Hope this helps,