If you are listed as a "joint" account holder then the account is effectively yours. It will help and hurt you in just the same way the account would if you were the sole account holder.
On the other hand, if you are listed as an "authorized user", then it will hurt you if the account contains negative information or if the utilization is high. Depending upon which FICO scoring model is used, it may help you if positive, otherwise it will not be used to score your report.
