Heroes is the plural form of hero, denoting more than one. Example: There were many heroes during Nine-Eleven. Hero's is a possessive form denoting something that belongs to the hero. Example: The...
The heroes have yet to be revealed, perhaps in book 2 (The Son of Neptune?) Jason, Percy, Annabeth, Piper, and Leo seem to be the first five in the group and Hazel and Frank the other 2.