Groupie notifies you whenever a major influencer orders from your store. The major benefit of this is that you can reach out to that customer and see if they're willing to endorse you in a tweet, blog, or send a photo of themselves using the product. This can get you direct traffic as well as links to your website which is the #1 factor for ranking in search engines.
Google's search algorithm has changed over the years. It's much better now to get quality links rather than quantity. A single tweet or link from a big influencer is better than a years worth of mediocre link-building.
Groupie will let you know if your customer has an IMDB page, Wikipedia page, a high Klout score, or a major following (Twitter, Instagram, Youtube) so that you can send them a personalized email.
Celebrity settings are completely customizable. You can adjust who should be deemed a "celebrity" (how many Twitter followers, how many Instagram followers, has a Wikipedia page etc).
This number will vary from shop to shop, but here are some approximate stats based on our current users.