The way I see it:
ISIS, or any other terrorist group, instructs muslims world-wide that it’s their duty to rise up and attack the infidels as their contribution to jihad. Lone wolves act on that and decide to commit an act of terror, be it an attack on one person (think of that poor Rigby fellow in Britain) or a larger plan (think Fort Hood).
Whether they were formally in communication with the terror group or not, they were carrying out their instructions, thus were part of the group.
Well stated, Judy. I think you might be right.