This fiddle adds a new item to the store when the button is clicked. And the grid will update itself to reflect the store's new contents.
The meat of the code is this:
var id = 0;
var button = new Button({
onClick: function () {
store.newItem({
id: id,
col2: "col2-" + id,
col3: "col3-" + id,
col4: "col4-" + id
});
id++;
}
}, "addRow");
Screenshot (after 2 button presses):