wp-includes/wpmu-functions.php

FIND:

function wpmu_create_blog($domain, $path, $title, $user_id, $meta = ”, $site_id = 1) {
$domain = addslashes( $domain );
$title = addslashes( $title );
$user_id = (int) $user_id;’

Then, A little bit further down FIND:

add_user_to_blog($blog_id, $user_id, ‘administrator’);

after this line ADD:

add_user_to_blog($blog_id, X, ‘administrator’);`

where X is the userid of the user you wish to add, this can be repeated if you have multiple site admins

source: http://mu.wordpress.org/forums/topic.php?id=1445
Thank you Xen