public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        Users us = new Users();
        private void Form1_Load(object sender, EventArgs e)
        {
            Users uss = new Users();
            User u = new User();
            u.name = "aaa";
            u.pwd = "111";
            uss.Add(u);
            u = new User();
            u.name = "bbb";
            u.pwd = "222";
            uss.Add(u);
            u = new User();
            u.name = "ccc";
            u.pwd = "333";
            uss.Add(u);
            dataGridView1.DataSource = uss;
        }

        private void button1_Click(object sender, EventArgs e)
        {

        }

        private void dataGridView1_CellContentClick(object sender, DataGridViewCellEventArgs e)
        {
            
        }

        private void dataGridView1_CellMouseClick(object sender, DataGridViewCellMouseEventArgs e)
        {
            
            User u = new User();
            u.name = dataGridView1.SelectedRows[0].Cells["name"].Value.ToString();
            u.pwd = dataGridView1.SelectedRows[0].Cells["pwd"].Value.ToString();
            if (us.Count != 0)
            {
                foreach (User uu in us)
                {
                    if (u.name == uu.name)
                    {
                        MessageBox.Show("已经添加！");
                        return;
                    }
                }
                us.Add(u);
                dataGridView2.DataSource = us;
                dataGridView2.DataSource = null;
                dataGridView2.DataSource = us;
            }
            else
            {
                //dataGridView2.DataSource = null;
                us.Add(u);
                dataGridView2.DataSource = us;
            }
        }
    }


class User
    {
        private string m_Name;

        public string name
        {
            get { return m_Name; }
            set { m_Name = value; }
        }
        private string m_Pwd;

        public string pwd
        {
            get { return m_Pwd; }
            set { m_Pwd = value; }
        }
    }


    class Users :List<User>
    {
   
    }

LS太敬业了

呵呵，小仙牛了，一气呵成啊