The simplest way would likely be to export your user list (admin > users ), import the pending users into an email service (like mailchimp for example), and send them a reminder. This takes some amount of reoccurring manual work.
For a more complex (but automated) setup, you could use webhooks and Zapier (See Configure webhooks that trigger on Discourse events to integrate with external services and Trigger a Zapier task with Discourse Webhooks).