Give PMPro members on one subsite membership on another subsite.

give_users_free_level_on_main_site.php

/**
 * When a user is given a level, give add that user to another site
 * in the network and give them a level there too.
 * PMPro should be active on both blogs.
 * The other blog should have the membership level already setup, note the ID.
 * Blog/site IDs can be found in the Sites page of the network dashboard.
 * This is not mean to work with the other PMPro multisite add ons.
 * Add this to a custom plugin or as a Code Snippet on your site.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
function give_users_free_level_on_main_site( $level_id, $user_id ) {
	// this is the site id and level id
	// of the other site to give the user a level on.
	$main_site_id    = 1;
	$main_site_level = 1;
	
	// if level > 0 give user free level
	if ( 0 < $level_id ) {
		switch_to_blog( $main_site_id );
		add_user_to_blog($main_site_id, $user_id, 'subscriber');
		remove_action( 'pmpro_after_change_membership_level', 'give_users_free_level_on_main_site', 10, 2 ); // prevent loops
		pmpro_setDBTables();	// make sure PMPro DB table names are set in $wpdb
		pmpro_changeMembershipLevel( $main_site_level, $user_id );
		add_action( 'pmpro_after_change_membership_level', 'give_users_free_level_on_main_site', 10, 2 );
		restore_current_blog();
	}
}
add_action( 'pmpro_after_change_membership_level', 'give_users_free_level_on_main_site', 10, 2 );