In a small bowl, dissolve yeast in 2/3 cup warm water. In a large bowl, combine mashed potatoes, sugar, shortening, eggs, salt, remaining 2/3 cup water, yeast mixture and 2 cups flour; beat until smooth. Stir in enough remaining flour to form a soft dough., Do not knead. Shape into a ball; place in a greased bowl, turning once to grease the top. Cover and let rise in a warm place until doubled, about 1 hour., Punch down dough; divide into thirds. Divide and shape 1 portion into 15 balls; place in a greased 9-in. round baking pan. Cover with a kitchen towel. Repeat with remaining dough. Let rise in a warm place until doubled, about 30 minutes. Preheat oven to 375°., Bake rolls until golden brown, 20-25 minutes. Remove from pans to wire racks. Serve warm.