VN is strict when using dining credits. Anyone using credits has to order their own main dish. You cannot share meaning you cannot use 1 credit to feed 2 or more people. If ordering pizza you can only order the personal size and you cannot combine credits to order a larger size. If you choose the Build it Yourself pizza a dining credit covers just the basic pizza and any additional toppings are paid out of pocket. You can also use credits to order any of the other entrees on the menu.
Once in a while someone will post that another restaurant or server would not allow sharing, but at most Table Service restaurants where you order from a menu you can share. It is a good way to save credits for a 2 credit meal, additional meals or if someone isn't very hungry or has a small appetite.
I would not count on this. Occasionally someone has posted that they were able to combine credits for a larger pizza, but more often people have said they were not allowed to do this.
Edited to add: At this point it sounds like your friends had a nice server who allowed it. If it is a new policy at VN we will see more posts about it, but in the meantime I would assume it was an exception.