This is not wpForo function nor wpForo part. User registration and email confirmation is being done by WordPress core functions. wpForo just add a nice form. Other processes are made by WordPress so you should ask this question to WordPress support forum. You may also search for such a plugin or solution in web.

@pdhgceoaf2 @robert A little late perhaps, but Post-SMTP (WordPress plugin) has the ability to resend messages. Unfortunately, it is not able to process messages for previously sent stuff, but going forward it could be beneficial. I use this plugin on my own sites.

Open the forum link in a private/incognito tab, enter their email addresses one by in the login/lost password function and send. If you have dozens or more to send then you have a bigger problem.