It depends on how you want to do it. If you have the dining plan and would not be able to use the additional credits, then you could book 2 separate reservations for the room where 1 person is leaving. However, that might mess up your discount for the last night if it is on a package that requires a multiple night stay for the discount.
If you have the dining plan and others could use the extra day's worth of dining credits, then just leave it as is. We have done this without problem and used Magical Express. We just made the Magical Express reservation for the person leaving for a day early.
If you don't have dining, then it doesn't matter. What you may want to do though if it is a package with tickets is have everyone in that room reserve the package with the # of days the person who is leaving early needs. Then just upgrade the tickets for the others in the room to add a day after they get there.