Some of these links are for actual rides; others are links to groups who offer a multitude of rides:
Claremont Cycling Club, via MeetUp - social / fast social
Mobbin' Monday - fast/social and race/train
the Bud's Ride - race/train
Psycho-Lists, via MeetUp - fast social
Erik's Night Ride - fast social
Claremont Senior Bicycle Group - social / fast social
the Sunday Coates Shop Ride - social / fast social
Ride Around Pomona (RAP) - social
Cycling Connection - social / fast social
Butts on Bikes, IE (BOBie's), via MeetUp - social / fast social
Inland Empire Women Cyclists, via MeetUp - social / fast social
Empire Bikes Shop Rides - social / fast social
the Cow Ride - fast social
Milly's Riders Sunday Ride - fast social
the Rose Bowl Ride - race/train
Velo Retro Ride - social / fast social
Montrose Ride - race/train
the Barry Wolfe Ride - race/train
S.B. / Goleta Criterium Practice - race/train
Beside those, most of the local shops host rides during the week, and often on the weekends, both road and dirt, and of varying ability levels. The best way to find out about them is to contact the shops directly. Check these links for more info:
Jax Bicycle Center, Claremont
I have divided these rides into three categories - social, fast social, and race/train. This is always a tricky thing to do as edges are often blurred.
Rides that fall strictly in the race/train category are the easiest to define - take this category literally, the rides are the fastest; most of the riders on these rides also race during the season and take their fast-paced training rides quite seriously. Don't expect regroups, although some, like the Montrose or Barry Wolfe rides [may] stop for flats or mechanical problems. The Bud's Ride, the Rose Bowl Ride never stop.
Social Rides are also fairly easy to define. Then again just what constitutes a social pace differs person to person. In general though, these are the most relaxed-paced of the rides.
Fast social rides are where the boundaries really tend to blur. While these rides will stop to regroup from time to time, they also tend to contain a competitive element - sprints and King of the Hill climbs, will often drive the pace up to higher speeds. There are some bragging rights to be gained on these rides, but their intensity is below the level of the pure race/train category of rides.
Many of the groups offer multiple rides during the week, each geared toward riders of different ability levels; check them out, there is sure to be something that fits your need.