We will be in Phoenix during the most temperate time of the year! As for Sedona it may get a bit more chilly with the possibility of snow because of the higher elevation. Other wardrobe consideration include the stabby cactus, the heated swimming pool, and impromptu sessions at the house with other attendees.
I recommend dressing in layers, like the locals. They suggest a windbreaker and a warm layer underneath with something on bottom. The nights can get chilly and the days can be hot, but typically it stays between 45 and 65. Bring a bathing suit, lounge wear, sunscreen, and anything you would want to wear for a photo shoot.
For shoes, try to bring something tougher than the fabric of a tennis shoe. This will help protect you from a stray jumping cactus if you get too close.