Perhaps autostereoscopic displays can be theoretically used by multiple people simultaneously, but it's not how the 3DS is currently implemented. The original versions of the 3DS required the player to keep the screen at the right angle to get the 3D effect; without doing that, it's just a blurry image, much like taking off the 3D glasses at a movie theater. The New 3DS (yes, it's actually called "New 3DS") has built-in face tracking so the display automatically adjusts to the eyes of the SINGLE person it believes to be playing, which is much more convenient, but still not perfect as it can lose track sometimes and takes a little jiggle to get back into place.

On both devices, any time more than one person is looking at the screen, the display has to be set to 2D mode or it will be blurry for at least one person.

Nintendo also puts a disclaimer in place that states 3D mode should not be used by individuals under 7 years of age. From what I've read, this is just out of an abundance of caution and there's no real evidence that the 3DS's autostereoscopy can harm developing eyes.