add_action( 'cmb2_init', 'yourprefix_register_repeatable_group_field_metabox' );
/**
 * Hook in and add a metabox to demonstrate repeatable grouped fields
 */
function yourprefix_register_repeatable_group_field_metabox() {

	/**
	 * Repeatable Field Groups
	 */
	$cmb_group = new_cmb2_box( array(
		'id'           => 'metabox',
		'title'        => __( 'Repeating Field Group', 'cmb2' ),
		'object_types' => array( 'page', ),
	) );

	$cmb_group->add_field( array(
		'name'       => __( 'Repeatable text', 'cmb2' ),
		'desc'       => __( 'field description (optional)', 'cmb2' ),
		'id'         => 'my_text',
		'type'       => 'text_small',
		'repeatable' => true,
		'on_front'   => false,
	) );

	$group_field_id = $cmb_group->add_field( array(
		'id'          => 'my_group',
		'type'        => 'group',
		'description' => __( 'Generates reusable form entries', 'cmb2' ),
		'options'     => array(
			'group_title'   => __( 'Entry {#}', 'cmb2' ),
			'add_button'    => __( 'Add Another Entry', 'cmb2' ),
			'remove_button' => __( 'Remove Entry', 'cmb2' ),
			'sortable'      => true
		),
	) );

	$cmb_group->add_group_field( $group_field_id, array(
		'name' => __( 'Entry Title', 'cmb2' ),
		'id'   => 'title',
		'type' => 'text',
	) );

	$cmb_group->add_group_field( $group_field_id, array(
		'name'        => __( 'Description', 'cmb2' ),
		'description' => __( 'Write a short description for this entry', 'cmb2' ),
		'id'          => 'description',
		'type'        => 'textarea_small',
	) );
}