Add users to groups by purchase history (woocommerce)

We provide this example implementation https://github.com/itthinx/groups-wc-completed-customer which adds the user to a group upon order completion. You could fork and extend it to cover the particular scenarios and conditions that need to be met when. As the membership is added based on order status, you could count the number of completed orders for the customer and do the group assignment accordingly.

As an alternative to hooking on woocommerce_order_status_completed, you could hook on woocommerce_order_status_processing if you want the assignment to be done earlier.