当DataGridView的数据源是一个DataTable时，绑定的DataGridView默认是不接受新行的。要解决这个问题，可以通过将DataGridView的AllowUserToAddRows属性设置为True来启用添加新行的功能。以下是一个示例代码：

using System;
using System.Windows.Forms;
using System.Data;

public class Form1 : Form
{
    private DataGridView dataGridView1;
    private Button button1;

    public Form1()
    {
        InitializeComponent();

        // 创建一个DataTable作为数据源
        DataTable dataTable = new DataTable();
        dataTable.Columns.Add("ID", typeof(int));
        dataTable.Columns.Add("Name", typeof(string));

        // 绑定数据源到DataGridView
        dataGridView1.DataSource = dataTable;

        // 启用DataGridView的添加新行功能
        dataGridView1.AllowUserToAddRows = true;

        // 添加按钮点击事件
        button1.Click += Button1_Click;
    }

    private void Button1_Click(object sender, EventArgs e)
    {
        // 获取数据源
        DataTable dataTable = (DataTable)dataGridView1.DataSource;

        // 添加新行
        dataTable.Rows.Add(1, "John Doe");
    }

    private void InitializeComponent()
    {
        this.dataGridView1 = new System.Windows.Forms.DataGridView();
        this.button1 = new System.Windows.Forms.Button();
        ((System.ComponentModel.ISupportInitialize)(this.dataGridView1)).BeginInit();
        this.SuspendLayout();
        // 
        // dataGridView1
        // 
        this.dataGridView1.ColumnHeadersHeightSizeMode = System.Windows.Forms.DataGridViewColumnHeadersHeightSizeMode.AutoSize;
        this.dataGridView1.Location = new System.Drawing.Point(12, 12);
        this.dataGridView1.Name = "dataGridView1";
        this.dataGridView1.Size = new System.Drawing.Size(240, 150);
        this.dataGridView1.TabIndex = 0;
        // 
        // button1
        // 
        this.button1.Location = new System.Drawing.Point(12, 168);
        this.button1.Name = "button1";
        this.button1.Size = new System.Drawing.Size(75, 23);
        this.button1.TabIndex = 1;
        this.button1.Text = "添加行";
        this.button1.UseVisualStyleBackColor = true;
        // 
        // Form1
        // 
        this.ClientSize = new System.Drawing.Size(264, 203);
        this.Controls.Add(this.button1);
        this.Controls.Add(this.dataGridView1);
        this.Name = "Form1";
        ((System.ComponentModel.ISupportInitialize)(this.dataGridView1)).EndInit();
        this.ResumeLayout(false);

    }

    [STAThread]
    static void Main()
    {
        Application.EnableVisualStyles();
        Application.Run(new Form1());
    }
}

上述代码演示了如何在DataGridView中添加新行。创建一个DataTable作为数据源，然后将其绑定到DataGridView。通过设置DataGridView的AllowUserToAddRows属性为True，用户就可以使用DataGridView的默认添加行功能来添加新行。在按钮的点击事件中，可以使用数据源的Rows.Add方法来添加新行。