When you stumble across Tenpenny Tower for the first time, you will see a ghoul arguing with the the guard at the front gate.

Talk with the residents and Mr Tenpenny and you will find that the ghouls want to stay in the tower.

Go find the ghoul and help him, and in return, he will give you a ghoul mask.

This mask will make you invisible to all feral ghouls, but do not shoot them while wearing it.