Примерно решение:

function solve(input) {
  let map = new Map();
  let neighborhoods = input.shift().split(', ');

  for (let neighborhood of neighborhoods) {
    map.set(neighborhood, []);
  }

  while (input.length > 0) {
    let current = input.shift().split(' - ');
    let neighborhood = current[0];
    let person = current[1];
    if (neighborhoods.includes(neighborhood)) {
      map.get(neighborhood).push(person);
    }
  }

  let sorted = Array.from(map)
    .sort((a, b) => {return b[1].length - a[1].length;});

  for (let neighborhood of sorted) {
    let neighborhoodName = neighborhood[0];
    let persons = neighborhood[1];
    console.log(`${neighborhoodName}: ${persons.length}`);
    for (let j = 0; j < persons.length; j++) {
      console.log(`--${persons[j]}`);
    }
  }
}