How to let registered users have access to all sites in MU?

They don’t have to re-register to post comments on all sub sites. Do you want tthem to be able to post comments, or actual blog posts?

Blog posts

Then you’ll have to register them as author on every site when they sign up.

I know dsader left a script in here somewhere…. ??

This?

https://www.ads-software.com/extend/plugins/wpmu-default-user-role/

In here or the wpmu forums from a while ago:

https://www.ads-software.com/support/topic/automatically-add-all-new-users-to-all-sites-on-network-as-subscribers?replies=15

https://www.ads-software.com/support/topic/add-new-user-to-all-blogs?replies=1

https://mu.www.ads-software.com/forums/topic/9913

Anyway, something like this works on my test server with only a handful of blogs:

<?php
function ds_add_new_user_to_all_blogs($user_id) {
  global $wpdb;
  $blog_list = $wpdb->get_results("SELECT blog_id FROM " . $wpdb->blogs);
  foreach ($blog_list as $blog) {
    add_user_to_blog($blog->blog_id, $user_id, 'author');
  }
}
add_action( 'wpmu_new_user', 'ds_add_new_user_to_all_blogs' );
?>

This adds user as author to all other blogs, while user gets admin role on his “gimme site” from signup.