TLDR: This seems like a strange feature of TSA Precheck. Anyone who is on the same itenerary as a person with TSA Precheck will also be awarded TSA Precheck status on their boarding passes-- no background check or other verification required.
Details: On a recent business trip with a co-worker I noticed that both of us had TSA Precheck status on our airline boarding passes despite only one of us actually having been enrolled in the program. We both got access to the reduced scrutiny TSA Precheck line.
Once I got home I contacted United through their bug bounty program (they issued the boarding passes) and they replied in a timely manner with:
"Thanks for contacting us. It is actually supposed to work this way. Family members and/or companions who are travelling on the same itinerary (with the same confirmation number) should also receive TSA precheck. Additionally, this is actually determined by TSA, not United Airlines.
Please let us know if you have any further questions.
United Bug Bounty Program"
Others have noticed this, but it seems to vary somewhat by airline. I have heard of this happening on the big airlines (United, American, Delta) and I can personally vouch that this was how it worked for United when I travelled last week.
Enrolling in TSA Precheck requires a physical visit to an office, a background check and fingerprinting. It seems strange to allow companions of TSA Precheck travellers to skip all this and get access to the TSA Precheck lines.