"At 27:43 you’ll hear Elon greet this woman by asking, “Stand-in for Sergey?” That’s a reference to his friend, Google co-founder Sergey Brin, according to Bloomberg. Apparently that Model X belongs to him."
Brin attended the Model X launch event, but he didn't approach the stage when the 4th Model X was handed over. Instead, a "stand-in for Sergey" took the keys from Musk.
Tesla CEO Elon Musk got Model X #01, while Tesla investor and board member, Steve Jurvetson, took delivery of Model X #02 (Jurvetson owns the first Model S, too).