Complex sentence: The Hamburg group centered around Helmuth Hübener, 16-year old Mormon, and began by listening to "enemy radio" broadcasts of the BBC and transcribing the news reports as leaflets, which they then disseminated.
Split into simpler sentences: The Hamburg group centered around Helmuth Hübener, a 16-year old Mormon. He and his friends listened to "enemy radio" broadcasts of the BBC and then transcribed the news reports as leaflets, which they then disseminated.