When someone registers on our WP site I would like to automatically add them to a specific CiviCRM group. I don't think a smart group would work here (if I could figure out the right search) because we want to use the group for emails and folks need to be able to unsubscribe.
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!