Codex tools: Log in
Stores the new users details in the database and calls wpmu_signup_user_notification to email the new user an activation link.
<?php wpmu_signup_user($user, $user_email, $meta); ?>
wpmu_signup_blog does the same thing except that it also stores details of the new blog registered.
wpmu_signup_user_notification sends an activation email to new users after they have completed the signup process, it includes their activation link.