index.cs

AddMap<Company>(companies => from company in companies
    select new 
    {
        CompanyName = company.CompanyName,
        UserCount = 0
        CompanyId = company.Id
    });

AddMap<User>(users => from user in users
  from companyId in user.Companies
  select new 
  {
      CompanyName = default(string),
      UserCount = 1,
      CompanyId = companyId
  });

  Reduce = results =>
      from result in results
      group result by result.CompanyId
      into g
      select new 
      {
          CompanyId = g.Key,
          CompanyName = g.First().CompanyName,
          UserCount = g.Sum(dto => dto.UserCount)
      };