To start with, you should probably eat something before you go, then eat some pizza toppings. This will please both you and your host.

Don't feel guilty or rude for preserving your health. One good tactic is to tell people you're allergic to wheat, which is probably the honest truth anyway. Another is to tell them that you've discovered that you have blood sugar problems, which is also true, and that you therefore need to avoid starch and sugar. People tend to become solicitous instead of difficult if you have a health problem.

Families can be tough to deal with, since any change from established routine is usually resisted. Maybe you could tell them you're at risk for developing diabetes and need to avoid sugar and starch. You're not lying, you're just playing up the truth.

Your friends and family will probably need educating on what constitutes lo carb, since people automatically go to low fat instead, but if you ask for help in managing your "condition" you might be amazed at how accomodating they'll become. Good luck!