I recently booked 4 plane tickets for the same flight for myself and three friends. I have a Known Traveler Number for TSA Pre-Check, but none of my friends do. When we printed our boarding passes at the airport, all four of us had TSA Pre-Check printed on the boarding pass. The same was true of the return flight. Was this because I had a KTN? Did it matter that I was the one who booked the reservation (i.e. if one of my friends had booked it but still entered my KTN for my ticket, would we all have gotten Pre-Check)?
Who gets these is a closely guarded secret but the answers and comments on Can others in my party go through TSA PreCheck? definitely suggest having someone with a KTN makes Pre Check happen for the others. Choice quotes:
it seems that there is a higher likelihood of the TSA pre going to some traveling on the same record as another KTN holder.
Yes, that's been my experience, as well.
I have a KTN #. My wife does not and is not even a U.S. Citizen but always gets TSA Pre Check
I have the exact same experience as @Dave
My wife has never applied but generally gets TSA Pre on her UAL boarding pass also when we book and fly together.
I have KTN and my partner does not. I have made three roundtrip flights over the last six months (domestic) and we both receive precheck on our tickets