Are the 12 of you all sitting together? If so, I definitely wouldn't upgrade. It's probably more worth your while to sit with your team - if you upgrade, you become a "lone wolf". Not that this is a huge team-building activity, but the other 11 probably would form a subtle bond without you. Not to mention the fact that there will likely be 1 stranger sitting amidst them in what would have been your coach seat.

If the 12 of you are all scattered about the plane and you are not sitting next to a coworker, I would upgrade without hesitation. Even if a couple of them notice your seat assignment, everybody understands how these things work, and they shouldn't feel any resentment towards you for using your miles. Chances are good that they also collect miles and would use them for the same purpose if they had the chance.

If there was a single boss or client in your group, I might try to upgrade him/her as well. Might be a good career move.