You can use red wine or beef broth for the beer. You'll get a different taste with each.
Use a beer that you like the taste of. Guiness was used in testing the recipe in the Pillsbury kitchens.
This recipe makes two pot pies. The prepared mashed potatoes are spooned down the center of each of the two crusts. The dry potatoes are mixed in with the soup that gets spooned on top of the potatoes.