Meet Los Angeles Swingers Online
Most couples in Los Angeles start online before they go to a club. The main sites let you search by location, see who is nearby, and message people who are looking for the same thing.
SDC
SDC has a large number of LA members and good event listings. A lot of club nights and private parties get posted there. You can see who is going and message them before you show up.
Swing.com
Swing.com (formerly SLS) is one of the bigger U.S. sites. Plenty of Southern California couples use it. The event calendar covers LA and the surrounding area.
Adult Friend Finder
Adult Friend Finder has a large volume of profiles in Los Angeles. It is more open than the lifestyle-specific sites, so you will see more single people mixed in with couples.
Kasidie
Kasidie is popular with couples who like club events and West Coast parties. It has a tighter community feel than the bigger sites.
Most people try one or two and stick with whichever has the most active members in their area. You can usually browse for free and pay if you want to message.