They patrol outside the barrier, and hand out invitations to the mirror mansion, “Mirrors.” Though they throw potentially-lethal invitations at any Puella Magi they see, they genuinely do want to invite guests to the mansion.
However, it’s suspected that they think of dead guests as more convenient, since they’re easier to carry.
A Familiar that waits outside the Manor of Mirrors handing out invitations. When it spots a Magical Girl, it throws a lethal invitation letter at her. That said, it genuinely wants to invite people to the Manor. Some suspect it just believes it is easier to carry the body than actually get a living Magical Girl to go inside.