Official comment
If the Smart Form is set to "Double Opt In" and the Contact does not confirm their double opt in they cannot receive any bulk email from you. You can send them one-off emails from the Contacts collection, but they would not receive the email with username and password if it is part of a sequence.
I would recommend using Double Opt In Optional or Single Opt In for opt-ins to your membership site. Either one allows you to have bulk email sent to them pending a double opt in.
ONTRAPORT creates a random password for the user; you send an email with the merge codes for the Username and Password to them. After logging in the first time they can change their password in the Customer Center in WordPress, or you can change it for them in the Contact record > Memberships section.