00-user-aware-of-post.php

<?php

// Using post title as the value, but it's just an example.
add_user_meta( $user_id, $post_id, $post_title );

01-post-aware-of-user.php

<?php

// User user email address a value but just an example.
add_post_meta( $post_id, $user_id, $email_address );

02-two-way-association.php

<?php

/**
 * Using this association will give you the ability to query for information 
 * both on posts and users and then work with the data accordingly.
 */
add_user_meta( $user_id, $post_id, $post_title );
add_post_meta( $post_id, $user_id, $email_address );