[Falstaff]

GABA is one of the local clubs that does many centuries and organized rides... http://www.bikegaba.org/

As mentioned, PBAA does four big rides a year, with El Tour de Tucson being the biggest ride in the area by far... http://www.pbaa.com/Home.html

This site lists many of the other local group rides (updated frequently)... http://www.aliciarides.com/WeeklyRides.html