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Sends an activation email to new users after they have completed the signup process, it includes their activation link.
To help clarify the purpose of $domain and $path, in the activation email their are used as so...
I suspect this this helps make the code more generic as there are three possible ways to setup WordPress MU that I am aware of...
<?php wpmu_signup_blog_notification($domain, $path, $title, $user, $user_email, $key); ?>
wpmu_signup_blog calls this function to send the email notification to the end user, the parent function handles the storing of the new user and blog details into the database.
wpmu_signup_user_notification does the same thing except that it is used to notify users who signed up for an account only.