Can I add someone to my flight reservation?

Long story short, I have to fly across the country for a few days, and my niece randomly decided that she wanted to come. I have TSA pre-check but she’s a minor and when I looked it up online, they said that TSA pre-check will come for her only if she’s booked on the same reservation. She can totally go through TSA herself, but we would obviously like to stay together if possible.