Consider hiking groups (e.g., Sierra Club, NY-NJ Trail Conference, Appalachian Mountain Club, or maybe some local groups). Bicycling clubs (Princeton Wheelpersons -- or whatever PC moniker they assumed, Central Jersey Touring Society, etc.). Check out events and classes at county parks and colleges. Consider volunteering (Volunteer Match, or similar sites, maybe a first-aid squad if you're into such). There may even be hope for ski clubs (Princeton Ski Club, or others). SEAS if you're into sailing, or want to be.

Nature clubs are wonderful, but most of the participants will be middle aged or older. Great people, though, if you just want to be around great people: NJ Audubon, local Audubon Societies, Native Plant Society of NJ, maybe even the NJ Invasive Species Strike Team (which likely has younger participants).