I don't have that much experience cooking them, usually I eat them at restaurants. In the Mediterranean countries they are usually prepared really simply, sauteed or simmered with oil and garlic -- mussels go really well with garlic. I'm not sure how you would handle that if you are supposed to keep them in the bag. Because if you overcook them they can get rubbery. Maybe just defrost them and then take them out of the bag? You could sautee some garlic in whatever oil you like to use, and toss them with it. The shells add a lot of flavor, that's partly why they're left in. Then you toss them over pasta, and you shell them as you eat. It's very a impressive presentation, but does take a bit of fiddling to get them out of the shell, so don't serve them the first time at a very formal meal! A lot of people put them whole into seafood stews too, with lots of other fish and shellfish. You need lots of napkins for that, and an extra bowl for the shells.
I've also had them in a tomato-based sauce, which is good but don't wear a light colored shirt.
With the shells the 2lb might not go very far, you could have a couple of friends over for pasta and that would take care of it.
Found this basic recipe on Chowhound; you would have to skip the mussel-cooking part and just toss the defrosted mussels for a minute or two to warm them.
Basics: how to steam mussels
Someone on the Chowhound boards recommended this, which sounds fairly low-cal if you use reduced fat coconut milk. Thai-style mussels with coconut milk and lemongrass
OK, now I want mussels too.