I'd play out one of these two scenarios:

1) As an earlier poster has suggested, take an alternative flight than the co-workers. 12 people from one company on one flight isn't good business, imo, particularly if you are the key people on the same project.

2) If you do fly the same flight, be very low key about your upgrade. If noticed, be off hand about how the airline did something nice for you because you had flown with them before - which happens to be the truth.