I know about the griffith park ride up la tuna on sundays at 8:30. I've done it a few times.
But I've seen other seemingly organized groups on a fairly regular basis either going up GP or coming or going from the park on Sundays. Info?
Also I've seen a bunch of guys on Sat? or Sun? can quite remember coming in to Burbank from I'm guessing Hollywood because I saw them speeding down Olive around Warner Brothers area. Again info?
I dont know how organized they are but there are a lot of guys training together in the area. Just jump on the back and ask if you can tag along for awhile. If they dont want you with them they will just drop you at the first opportunity.