In order to add this social icon you should edit GGT Composer Addon.
wp-content\plugins\ggt-composer-addon\includes\modules\ and edit file teammembers.php.
Find this string
"GitHub" => "github", and paste after it this one
"Xing" => "xing".
Now you have possibility to add Xing social link.
You can use this tip for adding other custom social buttons.