I lived in AZ for about 40 years (In Ohio now). A good website to hookup with riders who are into single track is www.arizonatrailtalk.com. They post rides all the time and are a pretty friendly group.
There is also www.ridingaz.com. They also post rides but are about 50/50 bike/quad riders.
Tons of awesome riding around prescott in the Bradshaw Mountains and around Camp Wood.
If you are a racer go to www.amraracing.com or www.whiplashracing.com. AMRA does more technical singletrack races and Whiplash is more about higher speed two track desert racing. Both are fairly well run.