If I have a guest token, can I use that to take family growth? This would grow my family, but wouldn't it also make the guest part of my extended family too?
All jokes to the side, the easiest thing to do is regard a guest token as a temporary worker. It functions in all ways as a regular family member, unless the card that generates it specifies otherwise. Once it has fulfilled whatever function (action) you've given it, it will disappear during the return home phase.