Nested shortcodes

You cannot use shortcodes like this. The parser would not read that like you want.

But there is a workaround: Hijack the shortcode dropbox-foldershare-hyno, run the callback function for the wp-members on the link and pass the result to the original dropbox-foldershare-hyno callback.

Sample code, not tested:

// wait until the other plugins are loaded
add_action( 'wp_loaded', 'wpse_100100_shortcode_magic' );

function wpse_100100_shortcode_magic()
{
    add_shortcode(
        'dropbox-foldershare-hyno',  
        'wpse_100100_shortcode_replacement' 
    );
}

function wpse_100100_shortcode_replacement( $atts )
{
    global $bvwidget;

    if ( isset ( $atts['link'] ) )
        $atts['link'] = wpmem_shortcode( array( 'field' => $atts['link'] ) );

    return $bvwidget->replace_shortcode( $atts );
}

Now you can use the shortcode [dropbox-foldershare-hyno] and pass a value for the link attribute that should be converted from WP-Members before Dropbox Folder Share gets its hands on it.