How is membership or leadership determined? How is a cell determined in this case?

In the case of the Klan (which you conveniently bracket to only after WW2) it, in fact was an organization with a single "Imperial Wizard" at the helm before being broken up in the 20's. The "distributed chapters" (themselves organizations with membership roles and defined leadership) were only "distributed" after the national organization was dissolved to bow out of a legal battle with the IRS.

There absolutely exist fans of ICP who have coordinated to cause violence, crime, and mayhem. (Though I don't believe that people should be prosecuted or persecuted for enjoying a goofy rap rock band)