Is it possible via some extension or modification to assign customer to a customer group based on the product he buys? For example if we have a "Membership 2015" product and we need to automatically assign customers who buy that product to the "Members 2015" customer group. This way we could set up a catalog price rule that would give discounts for any customers that belong to the Members 2015 group.
I ended up with using modified version of this extension: http://mageshopapps.com/shop/customer-membership.html
However here is another extension that should do the job without modifications: https://plugin.company/magento-extensions/automatic-customer-group-switching.html