using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public class GroupMember

    {

        public int MemberId { get; set; }

        public int GroupId { get; set; }

    }

    public static void Main()

    {

        List<GroupMember> groupMembers = new()

        {

            new() { MemberId = 1, GroupId = 1 },

            new() { MemberId = 2, GroupId = 2 },

            new() { MemberId = 3, GroupId = 3 },

            new() { MemberId = 4, GroupId = 2 },

            new() { MemberId = 5, GroupId = 3 },

            new() { MemberId = 6, GroupId = 1 },

            new() { MemberId = 7, GroupId = 3 }

        };

        Dictionary<int, IEnumerable<GroupMember>> groupMembersByGroupId = groupMembers

            .GroupBy(member => member.GroupId)

            .ToDictionary(gr => gr.Key, gr => gr.Select(member => member));

        foreach (var groupId in groupMembersByGroupId.Keys)

        {

            Console.WriteLine($"Group members in group with GroupId = {groupId}:");

            foreach (var groupMember in groupMembersByGroupId[groupId])

            {

                Console.WriteLine($"\tMemberId: {groupMember.MemberId}");

            }

        }

    }

}