Sometimes they let you in the turnstiles before opening time, but you will only go as far as the next rope. Each park has it's own opening show, so you won't go any farther than that. At Magic Kingdom (for instance) each area has a rope across it and until the announcement is made welcoming everyone, the CM stands there with the ropes and then walks back with the crowd. If you have an early breakfast, you can get into the park but you can't go and ride rides or anything like that.