Hi,
As Xavier said we'll develop a feature to be able to get such data easily.
Until then if you want to get a list of user names of who bought a product, you can run the SQL query below via phpmyadmin :
SELECT d.name,d.username,d.email FROM #__hikashop_order_product AS a LEFT JOIN #__hikashop_order AS b ON a.order_id=b.order_id LEFT JOIN #__hikashop_user AS c ON b.order_user_id=c.user_id LEFT JOIN #__users AS d ON c.user_cms_id=d.id WHERE a.product_id=XXX ORDER BY b.order_created;
You need to replace XXX by the id of the product and #__ by the prefix of your tables.