Invite a none elgg member by email in a subgroup should generate an error message

[oseg]

Group_tools plugin allows a group owner to invite new members in a group through their email addresses (directly or through a csv file) which is a very useful feature if you want to invite people which are not yet members of your elgg site.

However, you cannot invite external users into a subgroup: obviously these external users are not members of the parent group. In this case, the plugin should display an error message.

Suggested fix in lib/hooks.php

function group_invite($hook, $type, $return, $params) {
	$user_guid = get_input('user_guid');
	$group_guid = get_input('group_guid');
	$group = get_entity($group_guid);

	$parent = get_parent_group($group);

	// if $parent, then this is a subgroup they're being invited to
	// make sure they're a member of the parent
	if ($parent) {
		if (!is_array($user_guid)) {
			$user_guid = array($user_guid);
		}

		$invalid_users = array();
                $unknown_users = false;
		foreach ($user_guid as $guid) {
			$user = get_user($guid);
                        if (!$user) {
                            $unknown_users = true;
                        } else if (!$parent->isMember($user)) {
                                $invalid_users[] = $user;                                                                    
                                }
                            }

                if ($unknown_users) {
			register_error(elgg_echo('au_subgroups:error:invite_noelggmember'));
			return false;
                } else if (count($invalid_users)) {
			$error_suffix = "<ul>";
			foreach ($invalid_users as $user) {
				$error_suffix .= "<li>{$user->name}</li>";
			}
			$error_suffix .= "</ul>";

			register_error(elgg_echo('au_subgroups:error:invite') . $error_suffix);
			return false;
		}
	}
}