I am with the North Valley OET chapter. - It's mostly our chapter that works on that particular stretch of the PCT, but all chapters are welcome to join in with any chapters work parties/rides/events/etc.
I think, even if you don't get actively involved with all their stuff, it's worth it to join. It's not expensive, and they do a lot of work to keep all kinds of trails open to equestrian use all around the state.
(That's my little plug for them. ;) I mostly only help them out with the PCT clearing and maybe go on a couple rides/year.)