Showing custom fields for Guest Authors

I am using the User-Titles plugin to display an author’s title (ex: Associate Sports Editor) for a newspaper website. However, I’d like to give titles to Guest Authors as well. I added a custom field, guest-title, to the post type Guest Author, but how should I show this custom field value in single.php ?

I would typically use get_post_meta($post_id, $key, $single); but I do not know how to get the $post_id of the Guest Author associated with the current post.

Any ideas?

http://wordpress.org/extend/plugins/co-authors-plus/