A superorganism is a collection of individual organisms acting as a group. In the context of LessWrong, this usually refers to a collection of uploads.
Discussion of the dangers of a superorganism of uploads was discussed by Carl Shulman in "Whole Brain Emulation and the Evolution of Superorganisms". He describes several enormous advantages that superorganisms would have that would encourage their evolution. These advantages mostly revolve around the ease of duplicating and preserving individuals with beneficial traits. Any "mutated" copies that didn't mind being modified or deleted for the benefit of the group would quickly dominate the group. This superorganism would rapidly out-compete biological humans as well as more self-concerned uploads. Because it would be evolutionarily advantageous, the uploads goal structures may start to drift far away from ours. Therefore this possible result of uploading can be considered an existential risk.