I have a login on the homepage
That is why they go to homesite/wp-admin/.
To redirect a successful login you will need to drop something like this into your "mu-plugins":
<?php
function ds_login_redirect($redirect_to, $request_redirect_to, $user)
{
if ($user->ID != 0) {
$user_info = get_userdata($user->ID);
if ($user_info->primary_blog) {
$primary_url = get_blogaddress_by_id($user_info->primary_blog) . 'wp-admin/';
if ($primary_url) {
wp_redirect($primary_url);
die();
}
}
}
return $redirect_to;
}
add_filter('login_redirect','ds_login_redirect', 100, 3);
?>
Users without a blog must be subscribers of at least the main blog. Where will users without a blog go when they login? They see only a subscribers limited Dashboard and profile anyway. The SuperAdmin->Options allows you to designate an alternate blog as the "Dashboard Blog", wordpress.com calls theirs the "Global Dashboard".