I would assume it was a paragraph about membership itself being prosecuted. Parts of the US seem to have a similar concept https://www.legalmatch.com/law-library/article/gang-affiliat...

>Gang Participation: Some states, such as Georgia and Texas, make it unlawful for anyone to form, associate with, or profit from a street gang.

Germany has such a thing with membership in a criminal or terrorist organization in §129 stgb. Being part of such an organization is in itself a crime. So individual members dont have to commit any crimes them self to be prosecuted. How ever I only know of the paragraph being used against terrorists, as well as outlawed political associations.