Both national and international law recognize that membership alone in some organizations is an offense. I will bring some extreme examples. In the Nuremberg trials, in order to minimize the need for proof, it was decided that membership in the high German command itself was a crime. (How high should be "high" was debated, but then decided.) In Israel, membership in the SS is a crime, no need for further proof of actual personal acts. The principle here is that when you are a member of an organization that is known to be committing lawless acts, and you do not resign, you become associated with the acts of your own free will. I am only suggesting fines, which are amply deserved.
Four Libyan soldiers said killed in battle with Islamists in Benghazi (Reuters)
from the article: Settlers block West Bank roads to protest construction freeze