Borrowers who attempt to log in to Blend without having activated their account will see a "This account hasn't been activated yet. Please check your email for your activation link."
What you can do
Ask the borrower to check their email for an activation email. If a long time passed since the borrower applied, simply re-send an invitation email to the borrower and ask them to activate their account using the link in the email.
Note: Blend does not send an activation email automatically when the borrower gets this message. Lenders must send a new invitation if the activation link expired.