In my experience American Express (USA) additional cards can be used without activation. The important issue is how long will American Express allow the card to remain valid without a SSN or similar?
I have seen AU cards on regular US Amex Plat accounts (personal not business ones) remain valid for nearly 4-5 years without a SSN provided to Amex as far as I know. It did come with some Amex follow-up calls or letters (can’t recall which) over a year or two.
There are some US residents who have Amex AU cards for use by relatives resident in countries beside the US or for use by foreign relatives whose legal stay/visit in the US is of a sort where getting or having a SSN is not part of the picture.
There are even some US citizens who don’t have SSNs, and I have to assume that some of them have Amex AU cards.