An extra person does not always require another main dish. Look at what the meal already offers and add the part that will help it stretch comfortably.
Bread and a bean salad can support a soup supper. More cooked grain may suit a vegetable-and-tofu bowl, provided there is enough sauce and substantial topping to go around. A simple potato side can work beside a stew.
Avoid pouring water into a finished dish merely to increase volume unless the texture and seasoning still make sense. A larger quantity of thin sauce is not the same as another portion of dinner.
Check the new guest's dietary needs before improvising. Keep the addition simple enough that the original meal does not overcook while you make it. Welcoming another person is easier when the plan stays recognizable.