This Skript uses the 'member' prefix for permission nodes.
Make sure it is not occupied.

member.* - Allow full interaction within the server.
member.place - Allow placing blocks.
member.break - Allow breaking blocks.
member.interact - Allow interaction with the existing world. (Damaging mobs, interacting with chests/containers, etc.)

If the players do not have the member.interact permission, mobs will not target them (unless interfered by other plugins), as they can not damage the mobs themselves either.