Sherlock had been invited to lunch by a rich man in London. He went to the man's house and found that the lunch was actually a huge party and there were a lot of people around.

Dreadful, he thought. I hate large gatherings.
"I didn't know there would be so many... guests," he told the host.

"Oh, these aren't my guests. They're my employees..." the host replied, "...all except for one of them. He's the only other guest here besides you. I'm not going to tell you who it is. Do you think you can figure out who it is?"

"Definitely," Sherlock replied. "Let's all head into the dining room, and you can address the group."

They did just that, and after everyone was seated, the host stood up, greeted everyone, and proceeded to tell a joke. It was a bad joke, though. A really bad one. It was quite possibly the worst that Sherlock had ever heard, and John had given him some real boring groaners in his time that were better. But by the time the host finished telling it, Sherlock had figured out who the other guest was, and pointed him out, grinning.

"That's amazing!" the host exclaimed.
"Not really," Sherlock replied, and explained how he arrived at his conclusion.

How did he figure out who the other guest was?