Show a login/register link at the bottom of any post that uses PMPro's membership shortcode.

show_login_register_links_on_member_pages.php

/*
	Show a login/register link at the bottom of any post that uses the membership shortcode.

	Note this doesn't check for specific membership levels.
	
	Add this code to your active theme's functions.php or a custom plugin.
*/
function show_login_register_links_on_member_pages($content) {
	//bail if PMPro isn't loaded
	if(!function_exists('pmpro_has_membership_access'))
		return $content;

	//does this post use the membership shortcode?
	if(has_shortcode($content, "membership") && !pmpro_hasMembershipLevel() && pmpro_has_membership_access()) {
		$pmpro_content_message_pre = '<div class="pmpro_content_message">';
		$pmpro_content_message_post = '</div>';
		$sr_search = array("!!levels!!", "!!referrer!!");
		$sr_replace = array("", urlencode(site_url($_SERVER['REQUEST_URI'])));
		//get the correct message to show at the bottom
		if(is_feed())
		{
			$newcontent = apply_filters("pmpro_rss_text_filter", stripslashes(pmpro_getOption("rsstext")));
			$content .= $pmpro_content_message_pre . str_replace($sr_search, $sr_replace, $newcontent) . $pmpro_content_message_post;
		}
		elseif(is_user_logged_in())
		{
			//not a member
			$newcontent = apply_filters("pmpro_non_member_text_filter", stripslashes(pmpro_getOption("nonmembertext")));
			$content .= $pmpro_content_message_pre . str_replace($sr_search, $sr_replace, $newcontent) . $pmpro_content_message_post;
		}
		else
		{
			//not logged in!
			$newcontent = apply_filters("pmpro_not_logged_in_text_filter", stripslashes(pmpro_getOption("notloggedintext")));
			$content .= $pmpro_content_message_pre . str_replace($sr_search, $sr_replace, $newcontent) . $pmpro_content_message_post;
		}
	}


	return $content;
}
add_filter('the_content', 'show_login_register_links_on_member_pages', 5);

This recipe is included in the blog post on "Show a login/register link for non-members when using the [membership] shortcode." at Paid Memberships Pro here: https://www.paidmembershipspro.com/show-loginregister-link-non-members-using-membership-shortcode/