using System;

using System.Linq;

using System.Collections.Generic;

using Newtonsoft.Json;

public class Program

{

    public static void Main()

    {

      var subscriptions = new List<Subscription>

      {

          new Subscription(101, 45),

          new Subscription(101, 47),

          new Subscription(102, 47),

          new Subscription(102, 49),

          new Subscription(101, 43),

          new Subscription(103, 43),

          new Subscription(104, 43),

          new Subscription(104, 45),

      };

        var list = subscriptions

            .GroupBy(a => a.ClubId)

            .ToDictionary(g => g.Key,

                          g => g.Select(a => a.MemberId)

            .ToList());

        var json = JsonConvert.SerializeObject(list);

        Console.WriteLine(json);

    }

}

​

public class Subscription

{

    public Subscription(int memberId, int clubId)

    {

        MemberId = memberId;

        ClubId = clubId;

    }

    public int MemberId { get; }

    public int ClubId { get; }

}