Last year my spouse and I had a friend join us on a trip to HKG. I helped him out buy subsidizing his ticket with pay with miles from my account. He was on a separate pnr as a result. I had no problem getting him seated with us in the preferred seats. Our connecting flight from BDL to DTW even change a couple times and it kicked him to the back and I never had a problem having him moved up with us. We even had an opportunity to catch an earlier connection back to BDL when arriving back at DTW. The help desk after customs SDC'd all of us onto an earlier flight. Make sure that they link your companions pnr to yours!