there is a free-form field that notes can be added, and it can and is utilized. The issue is because it is free-form, it’s more of a ‘if an agent looks there’ kind of thing more than a ‘computer can read/understand it’ kind of thing. So you can have agent see that one PNR and another are ‘linked’ via a note, but the computer can’t, so anything automated doesn’t work. Companion CPU for one. IRROPS protection is another - meaning ‘linked’ PNRs don’t see each other when a flight needs to be rebooked, and aren’t prioritized together, so priority 1 person might be rebooked on the last seat of the next and last nonstop flight of the day, whereas person 2 who doesn’t have the same priority gets booked on a connection with an overnight layover since thats the next best available. This is actually more easily overcome, since you can call an agent, and they can put you both on the next flight with available seats, but then you both essentially lose any priority since by the time you rebook, anyone else on your flight has already been rebooked onto whatever alternate there is.

look - it’s not like the air carriers haven’t thought about or looked at what can be done, but more of a cost-benefit thing. Since this is standard industry practice, pretty much, it’s not like UA is being unique or stingy here - the effort to build an automated field that can handle this is too time and/or cost prohibitive, or just cant be built well enough to be integrated properly with the hundreds of other functions the airline system needs, and so isn’t worth the time/effort/cost needed. In reality, of the millions of reservations that are live on airline systems, it’s not a big enough issue where airlines need to solve it. It is a relatively minor inconvenience, in the scheme of thing.

interestingly enough, UA has made some progress on some more advanced functions -they didn’t have before - it used to be, for example, that a PNR once split (typically due to wanting to be put on the gate upgrade waitlist) used to lose all priority from the premier of the original on the return - and that functionality now seem to exist. Being able to add an additional pax onto an existing PNR is just not going to happen - on UA or any other carrier - likely in the foreseeable future.