First off, look at the Shibuya Station map (click to enlarge). Narita Express arrives on track 3 or 4, down a long hallway from the rest of the station. If you look at the map, there are actually several ways to get to Hachiko, but they aren't labeled very well. If you look closely at this last frame of this first video, you'll see you're directed to go downstairs.
|Shibuya Station map|
After that, you head downstairs, where you actually have to walk the entire length of another train platform to get to the desired exit. If you're using a Japan Rail pass, you have to go through the staffed exit and show your pass to the employee.
Once you pass the gate head outside and look for the crowd!