Add-on credit cards, also known as supplementary credit cards, are issued under a primary credit card and have the same features. They are useful for family members but can impact the credit score of the primary cardholder if there are delays or defaults in payments. It is important for add-on cardholders to inform the primary cardholder about their transactions and for the primary cardholder to guide them on responsible usage. Regularly tracking expenses and setting spending limits are also important considerations. The primary cardholder bears the ultimate responsibility for the add-on card.