After reading your post, I started writing this module.

When installed, at the edit of a product node, you will find a local task 'Group subscribe'. You can select the groups you want the member to be subscribed to after purchasing this product.

With conditional actions, you have to create a new predicate with trigger 'Customer completes checkout' and action 'Subscribe to the groups ordered'. When a customer completes the checkout of the product, he will be member of the groups selected.

Let me know if this is ok for you or you want something extra.

Greetz

Dieter