Unstructured groups can exist, but they do not emerge spontaneously. Mold cells exert a lot of effort to form an unstructured organism that is notoriously difficult to "decapitate". There is some nice mathematics (graph theory, random graphs in particular) about this. The bottom line is that spontaneously emerging trust relationships result in a power hierarchy, without anyone making any effort to this end. Also, if the participants are unaware of the process, it takes very little effort (especially, if some crucial information and enough computational muscle is available) to steer it and put the "right" people in positions of power. By forming trust relationships on the far ends of the trust graph, the "center of gravity" of the power structure is shifted very quickly, after a "tipping point" is reached.
Some tricks to leverage this phenomenon have already been successfully applied in engineered revolutions in Serbia, Georgia and the Ukraine. There are some people at Columbia University in New York, who specialize in this field.
I find it particularly amusing that it's the same university, where Asimov wrote his SciFi about psychohistory. Chalk up one more for science following fiction. ;-)