>I don't believe that a case has been made
>that the thing called "society" is an illusion.
Society is not an illusion. Treating society as a "thing" is an illusion, or rather a delusion.
>A group entity is an illusion because.... ????
"Illusion" is your word, not mine.
>Because it gives "rise to a certain class of
>category error." What class of category error?
In this case, a form of the fallacy of composition, i.e. considering a composite (or abstraction) as on the same ontological level as its components, and attributing characteristics of components to the composite itself. As in "the rights of society", or "the good of society", or "government action".