A good example for that is Uber. They initially focused on the tech community, organising events and – crucially – free rides for those attending. Those who used the service told their friends about the experience. The result was rapid growth and model that could be rolled out elsewhere. Another good example are the meetups/events of Nomadlist or Producthunt. https://nomadlist.com/meetups

https://www.producthunt.com/meetups