Well, in my opinion, it is poor etiquette to make your tablemates uncomfortable with an overly long pre-dinner prayer. If you generally offer a protracted, spoken grace before a meal and will be having tablemates who may or may not share your beliefs/practices, I think good etiquette would have you either ask your tablemates if they would be comfortable joining you, say grace as a family in a more private setting to ensure that you are not making your tablemates uncomfortable, or make your grace a brief bowed head/"bless this meal and those who prepared and serve it" prayer instead of the 5 minute dissertation that might be the norm when you are at home.
If you do not say grace, it would be polite to try to maintain a respectful silence during your tablemates' prayers, provided they keep it a reasonable length of time. After all, even if you are an athiest, you can still offer thanks/good thoughts for those who have prepared and are serving you the meal.