From the stationery tab
Click the stationery tab (if not already open) and scroll down to the groups section. Choose your style and click-and-drag it from the stationary tab to the board.

From the board
Click-and-hold or touch-and-hold anywhere on a clear area of the board and the Group menu appears. While still holding move your cursor or finger down to create a square group, or up to create a free-form path.

Tip. If you click-and-drag one of the rounded corners styles on to the board you will notice a free-form icon on the border. Click-and-hold this icon to redraw the border as a free-form path.