How do I move child to another Family Sharing group?

I've tried following instructions from the support article Move a child to another group using Family Sharing - Apple Support

Tried both from phone and Mac, but when clicking on "add member" I don't get the option to invite others or invite in person, it just takes me through to "create a child account"?

[Re-Titled by Moderator]

Original Title: Moving child account to another family