There are many factors to be considered when determining the cost of adding another person to your car insurance. Some of these factors would be how they will be using your car, if there are any tickets on their driving record as well as their age. If the person that is being added has tickets, then this could cause your premiums to increase substantially. Individuals under the age 25 are considered a high risk due to the lack of driving experience, this will also cause an increase in your premiums as well. If you choose to add one person after considering these options, the increase will take place in your future bills. It is best to contact your insurance company before making any decisions or if you may have any questions regarding this issue since they are better prepared to answer this question. These are just factors that you should consider.