You can add a user to the ticket CC field - this means they will be copied in on ticket email messages, even though they are not the main user associated with the ticket. They will only be copied on outgoing emails from Deskpro - the main ticket user must cc: them in their email software for them to see incoming messages.

You can't add CC agents - the equivalent function is adding an agent as a follower. Whether a follower will receive emails about the ticket depends on their notification settings.