Check the default bbPress role associated with a level and then assign it to a new member during PMPro checkout.

assign-bbpress-role.php

<?php // Do not include in your Customizations Plugin
/**
 * Check the default bbPress role associated with a level and then assign it to a new member during PMPro checkout.
 */

add_action( 'pmpro_after_checkout', 'pmpro_add_forum_role', 11 );
function pmpro_add_forum_role( $user_id ) {
	// $member_data = get_userdata( $user_object->ID );
	$member_object = pmpro_getMembershipLevelForUser( $user_id );
	$level_id = $member_object->id;

	$user_role = 'subscriber';
	$bbp_role = pmprobb_check_membership_level_bbpress_role( $level_id );

	if ( is_wp_error( $user_id ) ) {
		echo $return->get_error_message();
	}

	if ( ! empty( $_POST['role'] ) ) {
		$user_role = $_POST['role'];
	}

	$capabilities_array = array(
		$user_role       => true,
		$bbp_role        => true,
	);
	$updated_roles = update_user_meta( $user_id, 'wp_capabilities', $capabilities_array );
}


function pmprobb_check_membership_level_bbpress_role( $level_id ) {
	if ( class_exists( 'bbPress' ) ) {
		// get values
		$options = get_option( 'pmprobb_options_levels' );
	}
	return $options[ $level_id ]['role'];
}