Experimental code to change number of invite codes uses per level.

my-pmproio-codes-uses-per-level.php

<?php
/**
 * This is an experimental workaround for PMPro Invite Only
 * to assign a different number of invite codes depending on
 * the level a user checks out for.
 *
 * This experimental code has not be thoroughly tested and is
 * considered as a "use-at-your-own-risk" code snippet that
 * is NOT suitable for a live/production environment.
 *
 * This workaround uses switching between cases of
 * the constant and requires the third parameter for
 * the PMPROIO_CODES constant to be set to true if
 * and when defining the constant.
 *
 * If the constant is not set beforehand it will be defined
 * in the my_pmproio_codes_per_level function.
 *
 * Only levels that are set in the  array of the
 * global variable $pmproio_invite_given_levels will be
 * given a custom number of invite codes as set in the
 * $invite_codes_per_level array.
 *
 * Usage:
 * 1. [Optional] define PMPROIO_CODES with the third parameter set to true.
 * 2. [Required] set the levels that will be given invite codes in $pmproio_invite_given_levels
 * 3. [Required] Set number of invite codes per level in $invite_codes_per_level
 */

// [Optional] Set default value, third parameter must be true.
// define( 'PMPROIO_CODES_USES', 1, true );

global $pmproio_invite_required_levels;
$pmproio_invite_required_levels = array( 14 ); // Level 14 requires invite code

global $pmproio_invite_given_levels;
// REQUIRED: Set the levels that will be given invite codes.
$pmproio_invite_given_levels = array( 4, 5 ); // Members of these levels are given invite codes

function my_pmproio_codes_uses_per_level() {
	if ( isset( $_REQUEST['level'] ) ) {

		$level = intval( $_REQUEST['level'] );

		global $pmproio_invite_given_levels;

		if ( ! in_array( $level, $pmproio_invite_given_levels ) ) {
			return;
		}

		if ( ! defined( 'PMPROIO_CODES_USES' ) ) {
			// Set default value, third parameter must be true.
			define( 'PMPROIO_CODES_USES', 1, true );
		}

		// REQUIRED: Set number of invite codes per level here.
		$invite_codes_per_level = array(
			4 => 2, // Level 4 receives 2 invite codes
			5 => 3, // Level 5 receives 3 invite codes
		);

		foreach ( $invite_codes_per_level as $level_id => $level_codes ) {
			if ( $level === $level_id ) {
				/*
				This will only work if the constant was
				previously defined once-only with the
				third parameter set to true
				*/
				define( 'PMPROIO_CODES_USES', $level_codes );
			}
		}
	}
}
add_action( 'init', 'my_pmproio_codes_uses_per_level' );