Lydia can enter into the Neitherword by saying:

"Though I know I should be wary,
still I venture someplace scary,
ghostly hauntings,
I turn loose! Beetlejuice, Beetlejuice! Beetlejuice!!!"

BJ can go in the real world, too, if Lydia says his name three times In Neitherworld Lydia can do everything she wants, cause it's the place of the absurd.