Adam Neumann was flying high. Literally.

His office-rental giant WeWork was months away from being valued at $47 billion. Revenue was doubling annually. And Mr. Neumann was zipping across the Atlantic Ocean in a Gulfstream G650 private jet with friends last summer, smoking marijuana.

After the group landed in Israel and left the plane, the flight crew found a sizable chunk of the drug stuffed in a cereal box for the return flight, according to people familiar with the incident. The jet’s owner, upset and fearing repercussions of trans-border marijuana transport, recalled the plane, leaving Mr. Neumann to find his own way back to New York, these people said.

Since Mr. Neumann co-founded WeWork—recently renamed We Co.—with Miguel McKelvey nine years ago, he has led with unusual exuberance and excess. His combination of entrepreneurial vision, personal charisma and brash risk-taking helped the company surpass $2 billion in annual revenue, and made it the country’s most valuable startup.

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