Answer: Yes, you need the a. It's because (in this case) Marta and Juan technically are objects of the verb gustar. And because they are people, they require the personal a.
A literal translation would be "the apples please Marta and Juan" or "the apples are pleasing to Marta and Juan." It would be tempting to think the redundant les wouldn't be needed, but as a general rule gustar always uses an indirect object pronoun.
As a thinking exercise and not to use in speech, it might be useful to change the order of the sentence to put the subject first: Las manzanas les gustan a Marta y Juan. There it seems more obvious that the a is needed. Now, that isn't the way that sentences using gustar are usually constructed, but it does follow the common "noun, verb, object" pattern that we use nearly all the time in English and most of the time in Spanish. Gustar just seems different because the subject almost always comes last, and because the English equivalent (Martha and John like the apples) has the Spanish subject as the object, and vice versa.
