What kind of paddling do you want to do?
In California, there really aren't large numbers of dirtbags but they are out there.
If you paddle Class 5, camp at Casa Loma on the Tuolumne. It is right off of hwy 120, East of Groveland, where Ferretti rd hits hwy 120 (the eastmost of 2 times). Camp there for laps on Class 5 Cherry Creek run of the Upper Tuolumne.
Further South, there are a few dirtbags on the Kern River, but no real dependable meetup spot on weekdays. 10:00am on weekends at Riverside Park in Kernville, although the crowd is more Class 3-4.