Here is my own personal experience and a clarification.

If you are a Platinum card holder, you can add authorized users as EITHER Gold card or Platinum cards. It costs $175 to add up to 3 AU at the Platinum level and $45 to add up to 5 AU at the Gold level.

If you are a Premier Rewards Gold card holder, you can add up to 5 AU free of charge.

That is why you are seeing some people talk about "$45 for 5" and other people bragging about getting it for free. Nothing about this promo changes fees for AU additions. Standard fees are followed based on your card type.

I was able to, on one phone call, add the promo to both my Plat personal and my PR Gold personal and then add some AU. I was asked to give both SSN and DOB and didn't know the trick about politely refusing SSN at the time so I provided them. I was told it will NOT result in a credit pull but will be used to determine if that person has been blacklisted by Amex in the past. As such, they used the term "approved" to indicate success with the additions of my AUs since they passed that check. I have some very credible intel from a friend that one of his AU's had the account report on their CR as an open account, even though it did NOT result in a credit pull. So be wary of that.

In order to do all of that on a single phone call, I had to transfer back and forth between MR and Card Services. Started with MR to get the promo added to the Plat, then to Card Services to add the AUs. Then back to MR to confirm the promo was on my PR Gold, then back to Card Services to add AUs to that one, too.

One last data point of interest: my second MR guy said that because both of my cards are linked on one MR account, that enrolling my Plat automatically enrolled my PR Gold account for the same promo, thus there was nothing further that needed to be done. Makes me a little nervous, but gives me an easily arguable point if the miles don't post properly in the future.