Hi,
There is no specific documentation for the "user group after purchase".
But the feature is quite simple ; when you edit a product and set a value for the "group after purchase" ; when a customer will buy that product (and the order will be confirmed), the customer will be added in the selected Joomla user group.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.