Description
Stores the new user and blog details in the database and calls wpmu_signup_blog_notification to email the new user an activation link.

Parameters
$domain
(string) (required) Domain of the new blog (see explanation below).
Default: None
$path
(string) (required) Path of the new blog (see explanation below).
Default: None
$title
(string) (required) Title of the new blog.
Default: None
$user
(string) (required) New user's username.
Default: None
$user_email
(string) (required) New user's email address.
Default: None
$meta
(object) (optional) Other meta information.
Default: [blank string]
$domain and $path
To help clarify the purpose of $domain and $path, in the activation email their are used as so...

http://$domain/$path

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...

Sub-domains - http://blog.domain.com/
Sub-folders - http://domain.com/blog
Top level domains (using sunrise) e.g. http://domain.com
Return Values
(null)
Usage

<?php wpmu_signup_blog($domain, $path, $title, $user, $user_email, $meta); ?>


Family
Sibling
wpmu_signup_user does the same thing except that it only stores the new user details as is called when the user does not signup for a blog at the same time.

Children
wpmu_signup_blog_notification sends an activation email to new users after they have completed the signup process, it includes their activation link.


http://codex.wordpress.org/WPMU_Functions/wpmu_signup_blog