Complex sentence: About 45,000 Norwegian collaborators joined the pro-Nazi party "Nasjonal Samling" (National Union), about 8,500 of them being enlisted in the collaborationist paramilitary organization Hirden, and some police units helped arrest many of Norway's Jews.
Split into simpler sentences: About 45,000 Norwegian collaborators joined the fascist party "Nasjonal Samling" (National Union), about 8,500 of them being enlisted in the collaborationist paramilitary organization "Hirden". In addition, Norwegian police units helped arrest many of Norway's Jews.