incognito genius
Someone with a very high social IQ, or emotional intelligence, and who has acquired an enormous amount of social capital through befriending a large number of people. Typically someone who organizes numerous social gatherings, but who is not a professional event organizer or party planner. Also, a master of the "after party".
You just know that there are going to be some surprises at the Hallowe-en party cuz Stacey is such an incognito genius.