think it's that simple. I don't think the Illuminati is a group.
Rather, it's sort of an abstract concept. Illuminati is the plural of Latin illuminatus, "enlightened". So I take that as meaning many enlightened.
As I posted below George Washington wrote about the Illuminati. Obviously there isn't one group of the same people hanging around well over 200 years old.
Instead, I think there is information or knowledge passed down along some people in the world. This knowledge talks about how to have and or keep and gain power over people in the world. Think about it. Where is all the power in the world? Two places: money and government. If you control either or both of these to large extent you have tremendous power over people.
So I think there is a lose group of people that have this "knowledge" and work toward certain aims. Those aims may not always be the same between all people with the knowledge, but in general it's always about keeping power. And this whole thing goes back a very long time.
EDIT: I forgot one other way to control people, which is religion as I mention above.