Add all customers who have already bought a specific product to a user group

Hi,

Yes.
First, for future customers, you can use the "user group after purchase" setting of the product to set that up.
For existing customers, you want to create a mass action via the menu System>Mass actions, and select the type "order". There, you'll need one filter on the order_product column "order_product_code" being equal to the product_code of the product. And you'll want an action "change the user group" and configure the action to target the user group you want.
Once you click on the "process" button of the mass action, it will do it all at once.
Note that if you mess it up, it can have dire consequences. So I would recommend to make backups, and to use an action "display the results" first to check what "elements" are found when running the filter(s) of a mass action, before using an action which change things.
www.hikashop.com/support/documentation/167-massaction-form.html