Add author role to members at checkout. Add this to your active theme's functions.php or a custom plugin.

pmpro-change-role-at-checkout.php

<?php
/*
	This code will make members signing up for membership level #1 authors and make them subscribers when they cancel.
*/
function my_pmpro_after_change_membership_level($level_id, $user_id)
{
	if($level_id == 1)
	{
		//New member of level #1. If they are a subscriber, make them an author.
		$wp_user_object = new WP_User($user_id);
		if(in_array("subscriber", $wp_user_object->roles))
			$wp_user_object->set_role('author');
	}
	else
	{
		//Not a member of level #1. If they are an author, make them a subscriber.
		$wp_user_object = new WP_User($user_id);
		if(in_array("author", $wp_user_object->roles))
			$wp_user_object->set_role('subscriber');
	}
}
add_action("pmpro_after_change_membership_level", "my_pmpro_after_change_membership_level", 10, 2);
?>

This recipe is included in the blog post on "Give New Members Author Role" at Paid Memberships Pro here: https://www.paidmembershipspro.com/give-new-members-author-role/