Is it possible to offer free shipping on only some subscription groups, not all of them, within the Bold Subscriptions app?
No, there isn't a feature within Bold Subscriptions to set free shipping for specific subscription groups. Free shipping can be set up for specific products within the Shopify shipping profile, but this will not automatically update existing subscribers.
As a workaround for existing subscribers, you can make a slight change in your customer's account which will cause Bold Subscriptions to look for a new shipping rate. For example, you could update the customer's address or the quantity of the product. You can then revert this change so the correct information is properly stored along with the new shipping rate. Also, any change to the customers address from within Shopify admin will also trigger the shipping rate to be updated on the Subscription.
Note: If you would like to create fixed shipping rates in Bold Subscriptions, you can update your existing subscribers shipping rates at the same time. For more information, please visit our fixed shipping rate article here: https://support.boldcommerce.com/hc/en-us/articles/16114424490900-Fixed-Shipping-Rates-in-Subscriptions-for-Shopify-Checkout.