I haven't argued anything before that post, but this conversation about the semantics of the word organization means is interesting to me. To answer your question, I'd say Yes? Deadheads were a group of people associating with each other under common interest and intent. They didn't particularly have leaders or any hierarchical structure, but they gathered in locations of common interest (concert venues and the surrounding local) based solely on individual discussion and desire, participated in the event alongside and with the group, and almost everyone participating identified as a deadhead. I really don't understand the problem with them falling under the edge of the umbrella of the term organization.

They were an organization when viewed as an association or society: in this case a voluntary association of individuals for common ends. Deadheads were a distinct subculture in and of themselves, and I don't understand in what universe that wouldn't qualify. Keeping with the musician fandom, I'd say the same for the Juggalo's. Being on the outer edge of the Venn diagram is still part of the whole picture.