Theme Customize API Panel Concept

Fields always go in sections. Sections go in panels.

$wp_customize->add_panel( 'newsletter_panel', array(
    'title'    => __( 'Newsletter', 'twentytwelve' ),
    'priority' => 41,
) );

$wp_customize->add_section( 'newsletter', array(
    'title'    => __( 'Newsletter', 'twentytwelve' ),
    'priority' => 41,
    'panel'    => 'newsletter_panel'
) );

That will put a “Newsletter” section inside a “Newsletter” panel. All the fields should still be attached to the section.