To infiltrate ISIS they'd have to know how to pray like a Sunni beforehand, they do it 5 times a day. They'd also need to shed any dialect that'd give them away... can't get past entry level speaking a Kurdish accent or with a Sadr City twang.1/set 3 if he switch his uniform he can go infiltrate Isis by nobody know.
Then you'd have to join them in their actions, which would include fighting or killing Shia or Kurds.
IS, on the other hand, has done well to recruit from other groups and tell them to stay put and act as sleeper agents... it's how they seized a piece of Damascus recently.