When a new user registers a new post is auto created and assigned to the new user as a draft.

WordPress-add new post to user when registering

// -----------------------------------------------------------------
// Add Post On New User Registration and Assign to User Registration
// -----------------------------------------------------------------


add_action( 'user_register', 'gamma_new_user_post' );
/**
 * Add a new post when user registers.
 */
function gamma_new_user_post( $user_id ) {

    // Get user data.
    $user_data = get_userdata( $user_id );

    // Add new post here.
    wp_insert_post( array(
        'post_title'    => wp_strip_all_tags( sprintf( 'User %s has Joined %s', $user_data->user_login, get_bloginfo('name')  ) ),
        'post_content'  => 'Testing auto post creation.',
        'post_status'   => 'draft',
        'post_author'   => $user_id,
        'post_category' => array(1),
    ) );

}