The Clipper card is a debit card that is typically tied to a user's credit card.
Regarding card sharing, I read years ago about "some guy on the internet" who freely shared his Safeway Club card number (a grocery store loyalty program) with anyone. People would send him SASE and he would return barcode stickers of his card that people could affix to theirs. He liked the idea that Safeway's customer tracker would see "him" shopping in multiple states within the same day. Safeway said they didn't care (because it was just noise compared to all their data).