Hi there @voodoochill,
There isn’t a way to use the MailPoet interface to create a WordPress user. But it should be fairly simple, knowing their email address, to go under Users > Add New on your dashboard and fill out the information required there to create them as a WordPress user.
You just need to fill out their username, email address, and set their Role to author, and you can also have WordPress notify them of their new account.
Once they are added as a WordPress User this way, they should appear in MailPoet as subscribed to the WordPress Users list in addition to any other lists they were on. If this does not happen right away, you can go under MailPoet > Lists, and use the pop out menu under “WordPress Users” to “Force Sync.”