When someone says that they made dinner for you, does that mean that the person made food meant only for the person, or can there be other people who are eating the dinner that the person made as well?

Such terms in modern English are used loosely; but in your construct, the implication is that dinner was made especially for you. The possibility exists that the host might join you at the table, such as with a beverage only. That others may appear at the table is not implied at all.