Melt the butter in a slow cooker on high. When the butter is melted, add the soup and season with salt and pepper. Stir the soup, then add the mushrooms. Coat the mushrooms with the soup, add the lid to the slow cooker and cook on high, stirring occasionally, for 5 hours.
Remove the lid and continue to cook on high, uncovered, for 2 more hours to cook down the sauce and allow it to thicken. Continue stirring occasionally. A small amount of liquid should be present upon completion. Reduce the temperature to low or warm for serving during the party. The sauce will continue to thicken during this time.