Your credit report is a detailed report of your entire credit behaviour and history. The credit score, however, is just a numerical representation of that report.
If you are a guarantor for someone who defaults on their loan, it means you are vouching for a defaulter. Hence, you will be responsible for their dues, and your credit score will also deteriorate.
If you are the primary cardholder, making repayments of any add-on cards is your responsibility. If you fail to make the payments on time, it will hurt your credit score.