Welcome! Always good to see more Monteros in SoCal
The best bet to get some off roading experience is either go to OHV areas (for example the San Gabriel one near Azusa) and have some fun there.
In terms of groups, you can check on the various forums including this one, but also sites like meetup.com have plenty of groups going around for off roading.
![Smile :) :)](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)