Restrict post access to only certain members that has access to a members only post.

allow-post-access-for-specific-users-only.php

<?php
/**
 * Allow post access for specific members (users) only.
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
function allow_post_access_for_specific_users_only( $hasaccess, $post, $user, $levels ) {
	$allowed_user_ids = array( 1, 2, 3 ); // set allowed user IDs here
	$restricted_post_ids = array( 18, 93, 157, 265 ); // set your post IDs here

	// return if no access or no level required.
	if ( ! $hasaccess || empty( $levels ) ) {
		return $hasaccess;
	}

	// Only apply restrictions for specific posts
	if ( ! in_array( $post->ID, $restricted_post_ids ) ) {
		return $hasaccess;
	}

	// Only apply restrictions for specific users
	if ( ! empty( $user->ID ) && ! in_array( $user->ID, $allowed_user_ids ) ) {
		return false;
	}

	return $hasaccess;
}
add_filter( 'pmpro_has_membership_access_filter', 'allow_post_access_for_specific_users_only', 10, 4 );