I don't think you are obligated unless you signed something. I wouldn't.
Moreover, let's say there are 8 partners in the group and your ticket costs 400. That is only 50 bucks per partner and that is in pre-tax dollars, so in essence only about 30 bucks out of each of their pockets.
For you it is 400, and you are less likely to go about submitting it as a business expense to the I.R.S. I would tell them to piss off unless you signed an agreement about cancelling, which I seriously doubt you did.