public static IList<参加者> Create()
{
    // …前略
    }).ToList();

    int count = list.Count;
    var rnd = new Random();
    for (int i = 0; i < count; i++)
    {
        int src = rnd.Next(0, count);
        int dest = (src + rnd.Next(1, count)) % count;

        var tmp = list[dest];
        list[dest] = list[src];
        list[src] = tmp;
    }

    return list;
}