It is free to add additional driver(s) to your Spaceships booking. It can all be done via the Online Check-in Portal.
The online check-in must be completed 7 days before pick-up so until that moment you can add a driver, free of charge as Spaceships includes things like this for free. One of reasons why so many people book a Spaceships campervan: you can add an extra driver for free. Travelling together is more fun.