saturday mornings at the bicycle ranch in north scottsdale, they have groups, which one is very fast.
also there is a ride here(copied and pasted):
June 3 (Sun) Spin Cycle
Meet at 6:30 AM at Desert Breeze Park in Chandler (located on Desert Breeze Blvd. southeast of Ray & Rural Rds). Ride options of 30+ or 40+ miles and pace options from intermediate to advanced. Breakfast near the end of the ride. Contact: Glen Fletcher.
good luck, there are plenty of rides on the weekend and a few on the weeknights.