As an addition to what many have already said, a marinaded salad from canned veggies is always great. Take virtually any canned veg; to make it fancy use whole marinaded mushrooms, artichoke hearts, etc. But the base can be a cans of beans of any sort -- anything from green and wax beans to garbanzos to kidneys. Take your favorite Italian dressing (I personally use good seasons with good olive oil and balsamic OR a nut oil and sherry vinegar. If you're doing soutwestern, use your favorite salsa instead, or olive oil and lime juice.
This is hard to say, but as we get older more and more often we will find that not everyone at the table can eat the same food. Sometimes they simply have to live with it. A dinner party for 6 or 8, when no two can eat the same food is impossible. Sometimes we have to hope that our friends will understand and be charming and chat up the person next to them while they skip a course. Just make sure you have at least one nutritious dish your friend CAN eat, and s/he should be able to deal with skipping a course. Using this policy I've fed vegans and severe diverticulitis victims, allergies of all sorts. Just make sure your 'problem' guests know what is in the food.
You simply cannot cater to everyone's needs and desires. Many of my friends have given up cooking meals for friends for this reason, and to be honest with you, I'm close to it. It used to be a great joy to make pasta or pizza from scratch for friends, and they loved it. Now I find that someone doesn't eat dairy, another doesn't eat meat, some don't eat eggs, another cannot stomach tomatoes. Sad.
