A ustedes les gusta el chocolate?
It may help if you translate it a bit more literally, just for purposes of understanding. Instead of saying "Do you like chocolate?", try saying [temporarily] "Is chocolate pleasing TO you?" Then it is easier to see that the word "a" is analogous to the word "to" in the English sentence.
It's a different order for "Les gusta a ustedes". In Spanish, if the verb modifies the subject, at least in this case that pronoun, instead of the subject being the one who carries out the action, it doesn't make sense without that "a".