Select > All (both the grouped pattern and invisible square). Go to Edit > Define Pattern. Give your swatch a name in the dialog box.
It will now appear on the Swatches palette, just like a colour swatch. Move your grouped pattern aside and draw a large rectangle with the shape tool and fill it with your swatch.
Check along the edge of the repeat to make sure everything is aligned perfectly. A misaligned element will look partially or totally chopped in half because it will misaligned from its ‘mate’. If this happens just ungroup your elements, adjust their positions, and try again.