Temporary fix for https://wordpress.org/support/topic/commenting-bug-under-buddypress-groups-multisite/ so that users can wait for WP Idea Stream 2.3.4...

wp-idea-stream-custom.php

<?php
/**
 * Temporary fix for:
 * https://wordpress.org/support/topic/commenting-bug-under-buddypress-groups-multisite/
 *
 * Will be part of WP Idea Stream 2.3.4
 */

// Exit if accessed directly.
defined( 'ABSPATH' ) || exit;

function tempfix_group_map_meta_caps( $caps = array(), $cap = '', $user_id = 0, $args = array() ) {
	if ( 'read_idea' !== $cap || empty( $user_id ) ) {
		return $caps;
	}

	if ( ! bp_is_group() ) {
		$is_group_referer = 0 === strpos( wp_get_referer(), bp_get_groups_directory_permalink() );

		if ( ! $is_group_referer || empty( $_POST['comment_post_ID'] ) ) {
			return $caps;
		}

		$idea_meta = get_post_meta( (int) $_POST['comment_post_ID'], '_ideastream_group_id', true );
		$group     = groups_get_group( array( 'group_id' => $idea_meta ) );

		// It's not a group simply return the requested caps.
		if ( ! is_a( $group, 'BP_GROUPS_GROUP' ) ) {
			return $caps;
		}

	} else {
		$group = groups_get_current_group();
	}

	if ( groups_is_user_member( $user_id, $group->id ) ) {
		$caps = array( 'exist' );
	}

	return $caps;
}
add_filter( 'wp_idea_stream_map_meta_caps', 'tempfix_group_map_meta_caps', 11, 4 );