Can I automatically add someone to a Civicrm group when the register as a WordPress subscriber?
I would suggest creating a profile in CiviCRM to use for the Website registration form. Under Advanced Settings in the profile setup there are two things you want to do.


Add new contacts to a group. Here you can select the mailing list group. They will be added when they complete the profile.
Make WordPress account creation required.


After the form is complete they will have a WordPress account, be in your mailing group and have a contact record in CiviCRM. You can easily gather additional information about folks registering for your site.

Hope this helps!