If a customer is trying to sign up and claims they haven't received an email verification email then you can resend it for them.
You'll need to log in as the StackCP User, so head to:
Customers -> StackCP Users
Locate the customer trying to sign up by their email address and select:
Options -> Sign in
You should then be prompted with the Not got the email? Resend. Select this and the email address verification email will be resent to them.
The email is sent from the email address you’ve specified in your Please verify your email address email template setup via the Email Templates page.