$users->add()
Adds a single user or an entire second collection to the current collection
$users->add(Kirby\Cms\Users|Kirby\Cms\User|string $object): Kirby\Cms\Users 
Parameters
| Name | Type | Default | 
|---|---|---|
| $objectrequired | Kirby\Cms\Users<TUser>orKirby\Cms\Userorstring | 
				no default value | 
Return type
This method modifies the existing $users object it is applied to and returns it again.
Exceptions
| Type | Description | 
|---|---|
Kirby\Exception\InvalidArgumentException | 
				When no User or Users object or an ID of an existing user is passed |