When there is a large group of guests and you are not really close friends with the hostess, then do not offer. If the meal has included only your family and the host family in an informal atmosphere, then it is a nice gesture to suggest, “Why don’t you let me help you with the dishes? I’d be happy to.” Your hostess may decline because she doesn’t mind washing, because she doesn’t want you in the kitchen, or because she would rather spend time in the living room with you. Whatever the reason, do not insist if she declines your offer. On the other hand, if she accepts your offer, you may have an opportunity for private conversation and expressing your gratitude.