From what I was told... Persona-users can enter the TV normally, but someone without a Persona can also be pushed inside by a Persona user. When that happens the person's 'Shadow self' is forced out. If they accept it, it becomes their Persona. If not... the Shadow goes berserk and tries to kill them.

[Sounds incredibly welcoming, doesn't it?] Apparently Personas and Shadows are closer than we thought.