Redirect users to their blogs homepage

Hi, this is really a continuation of the same question posed a couple of months ago, but is now closed.
https://premium.wpmudev.org/forums/topic/how-to-make-login-redirect-take-users-to-their-blogs-homepage

In the last thread, David came up with a solution:
function login_site_redirect ( $redirect_to ) {
global $user;
$primary_blog_id = get_usermeta($user->ID, 'primary_blog');
$blog_details = get_blog_details($primary_blog_id);
$redirect_url = $blog_details->siteurl;
return $redirect_url;
}

add_filter ( 'login_redirect', 'login_site_redirect', 10, 3 ) ;

I tried adding this to my functions.php but it had no effect. Questions:
1. Did it need to be in a specific place in the functions.php file?
2. Does it need another closing tag after the last semi-colon?
3. Is the primary blog established when a user signs up for a new site? Or do I need to change sign-up to establish this?
4. My site owners are set-up as editors (should that matter) on their sites and subscribers on the main site?
5. When this works will it take them to their front-end or back-end dashboard?