The UX of adding new external users (no prior Zendesk account) via the CC list is quite cumbersome.

Scenario: Mid-ticket, a customer adds another person as CC to their reply (via e-mail). There is no warning or note about this in the Agent workspace - an agent has to "see" that someone new was added to the conversation; otherwise they won't receive the following reply in the ticket from that agent.
Now, even if the agent notices the new CC, they have to manually copy their e-mail address to the CC list, click on "Add User" and then have to fill in their e-mail address and name again.

At the very least, I'd expect Zendesk to remember the address of the new user that has just been copied to the CC list and carry that over to the "Add User" dialog.

At best, Zendesk should show a notification that new addresses have been added to the conversation by a customer and provide an easy way to add those users to Zendesk and to the conversation in the ticket without having to copy-paste the e-mail address several times.