SEA BRIGHT, N.J. — If you find the rare parking spot here, and climb one of the few public stairways over the seawall to a crowded patch of sand, you might notice a nearly empty expanse of beach nearby, beyond a “no trespassing” sign.

Step past the sign, and a young man might zoom up on an all-terrain vehicle to shoo you away, warning that this is the property of the Surfrider Beach Club. When questioned one recent afternoon, the young man said he knew nothing of the long-standing legal principle — and, more to the point, a 2009 court settlement — that appear to establish that the portion of beach he was patrolling is open to everyone.

Nearby, a group of friends who drove down for the day from Teaneck obeyed the sign, but it did not sit well. “Look at all that empty beach,” Samantha Soler, 20, said. “It doesn’t seem right.”

This is summer at the Jersey Shore, a low-cost escape for millions of people from several states, most of them unaware that they have stepped into perennial conflicts over how they can use the beaches.