Assuming your _dataGridView.Columns collection is not null and contains a template of the rows you wish to add, it is as simple as something like this:
foreach(var item in _collection)
{
_dataGridView.Rows.Add(item.Foo, item.Bar);
}
In order for this to work, you will have had to design your Columns collection in the VisualStudio designer or programatically add DataGridViewTextBoxColumn objects to the Columns collection.
In the example above, I added two DataGridViewTextBoxColumn objects to the _dataGridView.Columns collection and then populated the datagrid from a List of my object that contained a 'Foo' and a 'Bar'.
EDIT
Have you checked out the DataGridView FAQ? The information about using the DataGridView in unbound mode may help you.
HTH