Line the sofa seat with pillows so that it is high enough to allow guests to reach the table comfortably. If you are concerned about the appearance, experiment with putting the pillows underneath the sofa cushions.
Line the back of the sofa with pillows to put guests closer to the table. To enhance the appearance, consider using coordinating throw pillows.
Position the sofa along the wall in the dining room. This helps disguise the fact that you are using a sofa. It also allows people who are sitting in regular dining room chairs to slide them in and out. In addition, positioning the sofa against the wall economizes on space.