In shortcode form:

/**
 * Shortcode for listing all admin users of a Multisite site
 *
 * Usage: [siteadmins blog="1"]
 */
add_shortcode(\'siteadmins\', \'wpse_55991_site_admins\');

function wpse_55991_site_admins($atts, $content = null)
{   
    $site_admins = \'\';
    switch_to_blog( $atts[\'blog\'] );
    $users_query = new WP_User_Query( array( 
                \'role\' => \'administrator\', 
                \'orderby\' => \'display_name\'
                ) );
    $results = $users_query->get_results();
    foreach($results as $user)
    {
        $site_admins .= \'ID: \' . $user->ID . \'<br />\';
        $site_admins .= get_avatar( $user->user_email, 150 ) . \'<br /><br />\';
    }
    restore_current_blog();
    return $site_admins;
}


/* - - - - - EXTRA CODE - - - - - - */

/**
 * Shortcode for listing all admin users of all Multisite sites
 *
 * Usage: [allsiteadmins]
 */
add_shortcode(\'allsiteadmins\', \'wpse_55991_all_site_admins\');

function wpse_55991_all_site_admins($atts, $content = null)
{   
    global $wpdb;
    $blogs = $wpdb->get_results($wpdb->prepare("
        SELECT blog_id
        FROM {$wpdb->blogs}
        WHERE site_id = \'{$wpdb->siteid}\'
        AND spam = \'0\'
        AND deleted = \'0\'
        AND archived = \'0\'
        AND mature = \'0\' 
        AND public = \'1\'
    "));
    $site_admins = \'\';
    foreach ($blogs as $blog) 
    {
        switch_to_blog( $blog->blog_id );
        $users_query = new WP_User_Query( array( 
                    \'role\' => \'administrator\', 
                    \'orderby\' => \'display_name\'
                    ) );
        $results = $users_query->get_results();
        $site_admins .= \'Blog ID: \' . $blog->blog_id . \'<pre>\' . print_r($results,true) . \'</pre>\';
    }
    restore_current_blog();
    return $site_admins;
}