You don't need an affiliation thats just if you've got a fondness for a certain park or help with TM's there already you can list it but member at large is fine. Pay dirt is for racers, it helps get you a few extra points or something which can really help if its a close season. Other than that it doesn't really mean much for the non racer.
Careful what you say about not sure if donating would benefit you; some folks can go give you a lengthy reason why it would(either volunteer time or monetary)...Most NJ parks are north of you( Wharton, Estelle manor and Ceres are all pretty close to you though!) There are trail maintenance's every month at just about all of the parks so there is always some project to do at the parks.
JORBA helps keep all of their parks open to MTBers and they are always pushing for more access in more parks but they need more help both physically and financially to support all the efforts. I was surprised to learn how many parks where on the verge of banning bikers before someone stepped up and put out the fire. Complaints happen more often than you think about mountain bikers and its the work that JORBA does that helps keep our karma bank overflowing at a lot of these parks.
Anyway, i hope you decide to join its a great group of people. At the very least check out some of the trail maintenance sessions at the parks near you and get a feel for what its about.