On a Lumia 625, how do I add a new recipient to a message thread?

I find that I can only reply to one person when messaging in a pre-existing thread. So if I'm talking to 'A', and feel the need to include 'B', I see that it's not possible to add 'B' in the recipient box in the thread. I can't add someone new mid way. If I need to add someone, I necessarily have to create a new message adding the old recipient, as well as bringing in the new recipient.

Isn't this painful? Surely, there must be a fix?