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Sends an activation email to new users after they have completed the signup process, it includes their activation link.
<?php wpmu_signup_user_notification($user, $user_email, $key, $meta); ?>
wpmu_signup_user calls this function to send the email notification to the end user, the parent function handles the storing of the new users details into the database.
wpmu_signup_blog_notification does the same thing except that it is used to notify users who signed up for an account and blog at the same time.