Once you get the companion pass you can add the companion to any flight you are booked on as long as a seat is available on the flight. You can add them to a flight you booked 6 weeks a go or a flight you booked the day you get the pass. If you notice a flight only has a few seats left you could book the companion on points to hold the seat (if you have enough points) that way when you get the companion pass you could cancel the points booking and book using the companion pass. -Blake
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