Complex sentence: In the spring of 2008, Stardust Promotion added Momota to a recently founded idol group of school girls who performed on the walkways in Yoyogi Park in Shibuya, Tokyo.
Split into simpler sentences: In the spring of 2008, Stardust Promotion added Momota to their recently founded idol group. The group consisted of school girls and started its activities performing on the walkways in Yoyogi Park in Shibuya, Tokyo.