There are a lot of ways to make mussels in cream sauce.
Most of them start with sauteeing some leeks or shallots in a fair amount butter until softened. Sometimes, garlic is used as well. Sometimes an assertive herb like tarragon or thyme is used in moderation.
White wine or vermouth, less than you'd think because the mussels will produce plenty of liquid, is added to the aromatics. A few cooks like to use a splash of Pernod or ouzo to compliment the tarragon, if that's in the mix. The broth is brought to the boil, the heat's reduced to a hot simmer, and the mussels are added.
The mussels are cooked as soon as they open, plus one minute -- about five to seven minutes, total. When done, they're immediately removed and plated in large soup plates.
Cream -- from half and half to heavy -- is added to the broth. The ratio of cream to wine can be anywhere from 3 wine to 1 cream to vice versa. The richer the cream, the higher the ratio of wine. The lighter the cream, the lower the ratio of wine. The cream enriched broth is reduced, at a boil. The degree of reduction depends partly on the richness of the cream.
When the broth is sufficiently reduced and the flavors married, the taste is adjusted for for salt and (preferably white) pepper.
The broth is then poured over generously, but not so much as to completely submerge the mussels. All is garnished with lots of chopped parsley.
Empty bowls for shells should be set at table. These may be individual or communal, if communal there should be enough so they are easily reached.
A shellfish fork is not a traditional part of the place setting, a soup spoon is enough. The diners spoon their first mussel from its shell, then use the empty shell as tongs to remove the succeeding mussels. If you serve this way, you may want to explain -- even demonstrate. On the other hand, it may be safer to set a fork.
THE traditional accompaniment is frites. Condering the size of your party, that may be impractical, and you might want to consider some other way of cooking the potatoes, but not rice. You want something which can easily be plated alongside a bowl, which is also easy to eat -- preferably with fingers. If you're doing some sort of herbed, oven or sauteed potato. Either don't use an assertive herb in the mussels, or use a light touch with complimentary herbs on each dish. For instance, a little thyme in the soup, and the potatoes accented with rosemary and/or sage. If you have the oven space, lots of sheet pans, and the time, you could sandwich a thyme leaf between thin discs of potato (cut on a mandoline), and baked in a hot oven. The easiest thing might be to blanch potato spears in water until almost cooked, then saute them with herbs; or, still easier, dip them in olive oil, dress them with herbs and finish in a hot oven. In either case, the blanched potatoes could be held for quite awhile before cooking.
In order to simplify service, the mussels may be held in thier soup plates, without broth, and covered with cling wrap while the potatoes are cooked. When the potatoes are ready for service, the mussels are covered with hot broth (which brings them to temp) and served. If your group is divided into several tables, and the potatoes are partially precooked by blanching in oil or water, they can even be done in batches. In that way, you could manage frites -- which should be twice-cooked, anyway. A lot of work though, but it's how I'd do it back in the day.
Crusty French bread, whether sweet or sour is pretty much mandatory. I bake baguettes of pain de campagne.
It's very common, and a good idea to serve a green salad as a preceding course. The salad will likely be the only vegetable. You'll want something more substantial and interesting than a handful of watercress. You'll want to dress with something fairly acid -- like a vinaigrette. While there are any number of traditional French, Dutch and Belgian possibilities, I find a "classic" Caesar salad works very well with mussels -- or almost any seafood stew for that matter. If you do go Caesar, put the garlic in the salad and not in the mussels. A composed vegetable salad is also nice.
Since their sauce is broth, a soup course would probably not be a good choice to precede the mussels. Not only is it duplicative, your diners would have to get up from table too frequently to answer nature's call.
Even with the cream enriched sauce, the meal is fairly lean. You may want to begin with a rich meat or fish starter such as a pate, served with cornichon, etc. Go substantial but not creamy.
Dry Riesling and Traminers go well, whether the broth is made with vermouth or wine. So do Dutch and Belgian style beers. I would offer one wine and one beer.
Tarte tatin is a pluperfect finish, but there are lots of choices almost as good. Since there's such a thing as too much of a good thing, I'd stay away from anything "creamy."
Hope this helps,