Along with what was previously mentioned, there are 2 groups that operate primarily through meetup.com
CS 4x4
SCORCH 4x4
CS 4x4 is extermely friendly towards people new to the hobby and has monthly in-person meetings in Burbank where they gather and talk about rigs, previous runs, and lets people just meet face to face. They have runs of all experience levels. They also have do a number of weekend camping and family friendly trips.
I've only done one run with SCORCH and it was a little strange at times with the number of rigs, lack of organization at times, and how many people were just splitting off on their own throughout the run.