IPMS and AMPS, as you've now seen in the e-mail trail are both clubs for modellers. AMPS is only for armor, IPMS is for all modelling. I'm a member of both and live in Southern California.
Of the two, IPMS has more activity in Southern California right now. There was a stink at AMPS and much of the west coast people left. I'm hoping that'll change since AMPS is a cool organization too.
There are many IPMS chapters in Southern California so the best bet is to go to the national website (www.ipmsusa.org) and find the one nearest you. I'm a member of the Orange County chapter which claims to be the largest in the nation. I also know many in the Ontario chapter.
If you're wondering about joining, come to the Orangecon show and contest on October 17. The info can be found at
www.ipmsoc.org. That'll give you a great idea as to what IPMS is all about.
Wex