When you add someone to an email chain, explain why in a sentence. It tells the newcomer their role and tells everyone else why they're there. Without it, people get confused about who should be doing what.
Chapters
How to write great emails, tip number 11: Explain why you've added someone to an email chain. If there's an email chain with multiple people, with messages going back and forth, and you add someone to that email, put a sentence in that explains why you've added them. That is helpful to the person you've added— they know why they've been included— and it's helpful to everyone else on the email chain so they understand why you've brought someone else into the conversation. If you don't do this, the person joining the conversation is not going to understand why they've been added, and other people who are already there may wonder why you've brought somebody in. They may not understand the value that person's going to add or what they're going to contribute. So if you are involved in an email chain and you want to write a great email when you bring someone else in, add a sentence that makes it clear and explains why you've added someone to that email chain.