See this thread from 2017 for parallel promotion:
5000 MR for adding authorized user - Plat (USA) question
My recollection is that many people added three users, but received only one bonus. My
impression is that this tended to occur if the users were not real, but I do not recall if the thread explicitly confirms.
My recollection is people adding the users either on the same day or within a closely clustered days.
Online AMEX system requires SSN to add AU. On the phone the rep can take passport number, or with the tax ID provided later.
However if no SSN or acceptable ID doc info provided within the next couple months, the AU card would be canceled.
Given AMEX delayed posting of bonus, that means people do need to use real person(s) to get their AU bonuses.