It seems there are many ways to interpret what you are trying to do. Without more info, it is difficult to say what is allowed.
If you have the plan for 5 days but they are only staying for 3 days, it appears you have extra meals on your ticket. They paid for the tickets and got the free dining so you *should* be able to use them.
However, you talk about their saving money, which is where the confusion comes in. If you have the plan and are staying at a resort and they are staying with you without checking in, that's against Disney rules and they wouldn't get the dining plan (please don't think I am saying you are doing this, that is just an interpretation of what you have presented).
To get the free dining, everyone in your party would need to purchase the hotel room and the MYW tickets, so it doesn't appear they would "save" money by following the rules for the package, thus the possible interpretation.
In short, if all of you buy the room and the tickets and they just leave early, you probably can use their allotment of meals.
If they are just showing up and spending time in the hotel with you and purchasing their tickets separately, an interpretation possible with the info you have provided, they won't be getting free dining and you will risk getting caught and possibly asked to leave for violating the rules.