Commerce - How do I add the purchaser of a specific product to a user group? I want all buyers of a specific product to automatically be added to a specific user group.  

How do I do this?
Create a business logic plugin if you haven't already.  

Two good ways of doing that are : Business Logic Plugin or via http://pluginfactory.io

Then, in your plugin's main file in the init method, register a listener method:

public function init()
{      
    craft()-&gt;on('commerce_variants.onOrderVariant', 
        [
            craft()-&gt;businessLogic_logic,
            "onOrderVariantHandler"
        ]
    );
}


Then, implement the method (I do this in my class BusinessLogic_Logic Service):

public function onOrderVariantHandler($event){

    $variant = $event-&gt;params['variant'];
    if ($variant-&gt;product-&gt;id == "4194" ){
        $user = craft()-&gt;userSession-&gt;getUser();
        craft()-&gt;userGroups-&gt;assignUserToGroups($user-&gt;id, [3]);
    }

}


Obviously make sure you use the right ids for your particular products &amp; groups!

Note you can also just implement the method directly in init() if you want to keep things very simple - but the above method makes for a cleaner main plugin file and once you start doing a lot of this sort of thing (which is where a lot of the real power of Commerce lies I think) - separating the logic into a service and just registering the handlers is a more ordered way to go I think.

craft()-&gt;on('commerce_variants.onOrderVariant', function($event){
    $variant = $event-&gt;params['variant'];
    if ($variant-&gt;product-&gt;id == "4194" ){
        $user = craft()-&gt;userSession-&gt;getUser();
        craft()-&gt;userGroups-&gt;assignUserToGroups($user-&gt;id, [3]);
    }
});