public class Randomizer
{
    public static void Randomize<T>(T[] items)
    {
        Random rand = new Random();

        // For each spot in the array, pick
        // a random item to swap into that spot.
        for (int i = 0; i < items.Length - 1; i++)
        {
            int j = rand.Next(i, items.Length);
            T temp = items[i];
            items[i] = items[j];
            items[j] = temp;
        }
    }
}

private void Randomize_Click(object sender, EventArgs e)
{
    // Put the items in an array.
    string[] items = txtItems.Lines;

    // Randomize.
    Randomizer.Randomize(items);

    // Display the result.
    txtResult.Lines = items;
    txtResult.Select(0, 0);
}

// Assign the people to groups.
private void btnAssign_Click(object sender, EventArgs e)
{
    // Get the names into an array.
    int num_people = lstPeople.Items.Count;
    string[] names = new string[num_people];
    lstPeople.Items.CopyTo(names, 0);
    
    // Randomize.
    Randomizer.Randomize(names);

    // Divide the names into groups.
    int num_groups = int.Parse(txtNumGroups.Text);
    lstResult.Items.Clear();
    int group_num = 0;
    for (int i = 0; i < num_people; i++)
    {
        lstResult.Items.Add(group_num + " " + names[i]);
        group_num = ++group_num % num_groups;
    }
}