Reserve card additional users are free, And they are receive complimentary SC access'
Delta Sky Club Access
Delta SkyMiles® Reserve Basic and Additional Card Members receive complimentary access to the Delta Sky Club when traveling on a same-day Delta-marketed or Delta-operated flight. Reserve Card Members traveling on a Delta partner airline flight that is not marketed or operated by Delta may access the Club at an exclusive per-visit rate of $39 per person, per location.
The vanilla Amex Platinum charges $175 for each "authorized user".
As does the Amex Delta Reserve card, I believe.