The Key Words are "Plan Ahead"
As a Guest
Be sure to mention in advance any allergy that could cause a serious reaction. When you call to RSVP, explain to your host, “We’d love to come for dinner, but I have to tell you that Caroline has a serious shellfish allergy.”
Dietary preferences such as vegetarianism or a low-carb diet should be handled differently. At large parties and buffets, where there’s usually a large variety of foods to choose from, make do with what is available. However, if the gathering is small, the dinner is in your honor, or you’re going to be an overnight houseguest, mention your restrictions and offer to bring a dish to share. Say, “Thanks so much for the invitation. I should let you know that I’m a vegetarian. I’m happy to bring a quiche if that’s okay with you.” This way, your host won’t waste time preparing the wrong food for you or have to trouble himself figuring out what type of dish would best suit your preferences.
As a Host
It's important to accomodate any allegies that your guests may have--health and safety always come before menu preferences. If it's the first time you are having new friends to dinner, it's gracious to ask if there are any allergies or dietary restrictions you should know about. Make welcome guests who are vegetarian, vegan, or have any other dietary restrictions or preferences by ensuring there is enough for them to eat. They don't have to be able to eat all of it (you can still have a turkey at Thanksgiving if a vegetarian is coming to dinner) but they need to be able to participate in the meal.