We have launched an app that will automatically send account invites to those customers who are added to your store in a 'disabled' state. This is most often when a customer checks out as a guest on your store. Our app allows you to specify the number of days after an order to wait before sending the account invite. It's quite simple, yet effective.
App Store Link: https://apps.shopify.com/automatic-account-invites
Please send us any feedback or issues you might run into here: https://deeko.com/pages/automatic-account-invites-support