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.
Right now I’m not sure what the real use case and issue is here.
Above mentioned functionality is also documented here: https://user-docs.zammad.org/en/latest/basics/service-ticket/follow-up.html#responding-to-individual-messages