Assign buyer to specified user group after payment

[Sulpher]

It's very important ACL feature in situations when we need to restrict access to different products. For instance, I want to sell digital goods and provide personal support for each of them. I'm using Digicom and Kunena. I can create new user groups according product names and set permissions in Kunena categories. My request is to specify in which user group the buyer can be assigned in after the payment.