Complex sentence: The celebration is organized by GoTopless, a US-based organization with chapters in other countries, and is sponsored by Raëlian believers, a UFO religion founded by Claude Vorilhon.
Split into simpler sentences: The celebration is organized by GoTopless, a US-based organization with chapters in other countries. GoTopless is founded and sponsored by Raëlian believers, a UFO religion established by Claude Vorilhon.