[worldruined]

Originally Posted by Tornadium

Is there any information in the novels regarding membership in the organisation?

Off the top of my head... not much.

Spoiler for Volume 1:

"Just as you have guessed, the 'Organization' was founded three years ago, and their priority is to observe Suzumiya Haruhi. To put it bluntly, they exist solely to observe Suzumiya Haruhi. I'm sure you understand by now, right? I'm not the only 'Organization' member here in this school. There have already been a number who have infiltrated here before me; I've just been temporarily transferred here to assist them."

Spoiler for Volume 8:

"If you call him an associate then he is an associate, but he's an associate in a different sense from Arakawa-san and Mori-san. He isn't necessarily a part of the 'Organization'."

Koizumi glanced at the President, who had been puffing the smoke of his second cigarette up to the ceiling,

"He is our agent within the school. To talk a little bit about his motives, he provides us assistance in exchange for certain conditions. If Mori-san and I are part of the inner sanctum, then he is of the nave."

Although Koizumi mentions a "number" of Organization agents, we really only learn the identity of one for sure, and he's more of a mercenary than a "true believer". I think we can safely assume that he, at least, isn't an esper.

Speaking of said person:

Spoiler:

At this moment, Koizumi spoke as well,

"In that case, maybe I should bring a friend as well? In fact, I know someone who's interested in our club..."

I shut him up before he could finish. There's no need to bring your buddies over, they're going to be freaks anyway.

From Boredom; Koizumi mentions a "friend" but this is never elaborated on again. Might he have meant the (eventual) Student Council President, before deciding that the guy would be better off in the President role?