I've j/s on US many times. You only need to go to the gate and the agents will be happy to help you. They always have been with me. One thing however, when you get down to the airplane, the F/A's seem to take it upon themselves to block the flight deck, ask for your ID and J/S form and take it to the capt instead of you doing it yourself. The first couple of times I found this practice to be mildly annoying, but it's their rules and the were otherwise friendly and accomodating.