$pages->add() for $site->users()?

Does the add function exist for the Users object? I cannot see anything in the docs, and thought it might’ve been omitted. Or is there a way to achieve a similar thing with user collections?

No, add() does not exist for other collections. You could try $collection->append().

Or mimic the add() method:

 $users->data[$additionalUser->username()] = $additionalUser

But $users->append($additionalUser->username(), $additionalUser) should be sufficient in most cases, I guess.