The phrase "additional cardmember" refers to someone who is authorized to use a credit card or access a card account under someone else's account. Full definition
One additional cardmember benefit: You get eight annual barista picks, where a Starbucks barista curates and automatically uploads a food or beverage rewards to your account. (creditcards.com)
Purchases on additional cardmember cards can also be credited, but only up to a total of $ 200 per year for all cards on the account. (upgradedpoints.com)
Add an additional cardmember to your account and you'll earn another 5,000 bonus points when they make a purchase within the first 3 months. (pointsavvy.com)