RoliThCoder

using System;
using System.Collections.Generic;
using System.Linq;

class RoliTheCoder
{
    static void Main()
    {
        var input = Console.ReadLine();
        if (input == string.Empty) return;

        var eventsTable = new Dictionary<string, List<string>>();
        var idNameTable = new Dictionary<string, string>();

        while (input != "Time for Code")
        {
            var inputData       = input.Split('@').Select(x => x.Trim()).ToArray();
            var participants    = inputData.Take(inputData.Length).Skip(1).ToList();

            var idName = inputData[0].Split(new[] { '#' }, StringSplitOptions.RemoveEmptyEntries).Select(x => x.Trim()).ToArray();

            if (idName.Length < 2) goto Next;

            var id   = idName[0];
            var name = idName[1];

            if (!idNameTable.Keys.Contains(id) && !idNameTable.Values.Contains(name))
            {
                idNameTable[id] = name;
                eventsTable[name] = participants;
            }
            else if (eventsTable.ContainsKey(name) && idNameTable[id] == name)
            {
                eventsTable[name] = eventsTable[name].Union(participants).ToList();
            }

            Next: input = Console.ReadLine();
        }

        var eventsChart = from pair in eventsTable orderby pair.Value.Count() descending, pair.Key select pair;

        foreach (var pair in eventsChart)
        {
            var sortedParticipants = from list in pair.Value orderby list select list;

            Console.WriteLine($"{pair.Key} - {pair.Value.Count}");
            foreach (var participant in sortedParticipants)
            {
                Console.WriteLine($"@{participant}");
            }
        }
    }
}