Wifi Direct Support on iOS Devices
Theoretically the device list is as follows, running iOS 7 or later:


iPhone 5 or later
iPad (4th generation or later)
iPad mini
iPod touch (5th generation)


That said, this is the compatibility list for AirDrop which uses WiFi Direct as of iOS 7. Your mileage may vary with apps that are not utilizing AirDrop for the WiFi Direct functionality.