By default or just in specific cases?
Currently Zammad does not have such a functionality - most common use case is to also notify your supervisor (if such a request stands in the room). If this is your requirenment as well, I personally would rather suggest to provide your supervisor an Zammad account where he/she can follow the conversation in a better way.
If you just need it in some situations, just search for the user in the cc field or provide a mail address youw ant to include in the cc. If you receive mails with pre filled “cc” addresses beside the “to” field, use “reply all” to automatically fill the field.