hi Cordel, i have a friend who's been gluten/dairy free for nearly 2 years now, due to health issues, and you're right: if it's not something you live with all the time, it's pretty tricky. all of the sauces/soups/thickening agents that involve a pinch of flour or a splash of milk or a pat of butter, even when those things are far from the main ingredients, are right out.
but when you get used to it, there's plenty to go with. for example, most people with dairy issues have specifically cow-origin dairy issues. so a plate of sheep and/or goat cheese, with rice crackers (only wheat has the gluten in it, so any non-wheat bread-like products are fine) can be a nice treat for someone who hasn't had cheeses & crackers in a while. check with your party organizers to see if the non-cow thing is the only dairy to avoid.
fruits & vegetables are great options. fruit salad with a honey-vinaigrette dressing instead of a creamy one would be good for a dessert choice, as would a non-dairy fruit sorbet. pile on the fresh veggies into a hearty green salad, again with vinaigrette dressings instead of creamy ones. veggies roasted/sauteed/grilled in olive oil & savory herbs work out great as well.
any style of meat/fish/chicken dish you already regularly make would be fine, as long as you're not battering it in flour or cooking it with butter.
if someone in your group is really sensitive, keep a sharp eye on the ingredient list in any prepared sauces. soy sauce, which most of us just think of as liquid salty, generally contains some wheat, for example.