If the restaurant doesn't have a credit card hold don't worry about it. Just go and say some couldn't make it.
If the restaurant has a credit card hold, we don't yet have anything but conflicting information from WDW-DINE regarding whether or not a partial party will be charged the no-show fee for the guests who don't show up. So at this time, I'd suggest that you treat it as though you WILL be charged for them, and try to remove them no later than 48 hours before the reservation. You also might get different answers from WDW-DINE as to whether or not they can just remove people or whether they'll have to cancel and rebook for a different party size. Sorry there isn't a better answer here.