Unfortunately there's no such feature built into Flarum.
If an email didn't arrive, the user is expected to login and click the re-send link in the banner at the top of the page, or use the password reset page.
One option would be to extract all email addresses and manually send a message from an external email client or newsletter generator to send the link to the password reset page to every affected user.
You can visit a user profile to manually approve the account. But you'll never know if the email was really valid in that case.