Extranet::isUserInGroup

static function isUserInGroup($userId, $currentUserId = null)
{
	$currentUserId = BitrixImCommon::getUserId($currentUserId);
	if ($currentUserId <= 0)
	{
		return false;
	}

	if ($userId == $currentUserId)
	{
		return true;
	}

	$extranetUsers = [];
	$groups = self::getGroup([], $currentUserId);
	if (is_array($groups))
	{
		foreach ($groups as $group)
		{
			foreach ($group['USERS'] as $uid)
			{
				$extranetUsers[$uid] = $uid;
			}
		}
	}

	return isset($extranetUsers[$userId]);
}