How do you add another person to a private message when it's already sent?

How do you turn on the ability to add a person to a private message?

I’ve just been alerted by one of our users that they cannot add someone to private message. On investigation, this is replicated in my test accounts in that forum. My admin account still can.

My other Discourse forums do not have this problem. Safe mode doesn’t resolve it. I can’t see any setting that I’ve changed which prevents people being added to a PM.

Am I missing something?