If you are using an unbound DataGridView, you can create new rows and then add them to DataGridView.
// Sample code to add a new row to an unbound DataGridView
DataGridViewRow YourNewRow = new DataGridViewRow();
YourNewRow.CreateCells(YourDataGridView);
YourNewRow.Cells[0].Value = "Some value"
YourNewRow.Cells[1].Value = "Another value";