How to list users by custom field?

To fetch users with certain meta field, you can use WP_User_Query class. Check codex manual for it.

So you can create a custom shortcode like this:

add_shortcode( 'listusers', 'wpse8170_listusers' );
function wpse8170_listusers( $atts, $content="" ) {
    $atts = shortcode_atts( array( 'company' => false ), $atts );

    if ( empty( $atts['company'] ) ) return $content;

    $query = new WP_User_Query( array(
        'meta_key' => 'company', 
        'meta_value' => $atts['company'],
    ) );

    ob_start();

    // render your users list here

    return ob_get_clean();
}