getControllerGrip( index ) {

		const controller = this._getController( index );

		return controller.getGripSpace();

	}
Returns an instance of `THREE.Group` that represents the transformation
of a XR controller in grip space. The requested controller is defined
by the given index.

@param {number} index - The index of the XR controller.
@return {Group} A group that represents the controller's transformation.