You can quickly pull a list of people who have incomplete addresses under People > Advanced search > Address by clicking on the check box for whether the address is "mailable/mappable."
You can use one of the following links in an email blast for a user to update their address:
Without a custom domain: http://yournationslug.nationbuilder.com/forms/signups//edit
With a custom domain: http://yourdomain.com/forms/signups//edit
Note: this will only work if the user signed up to the nation with an email address