Spitfire is a virtual whiteboard to be shared by multiple persons at the same time. Users will be anonymous - so no one needs to sign in.
Currently you can access it on spitfire.peewee.space and start a whiteboard session.
Share the URL of your session with the people you want to work with. Anyone who has access to that URL can participate in your work.
Spitfire is free to use for everyone. It is developed on top of the Meteor platform while obeying the agile mantra: Build the simplest thing that will possibly work. Validate. Repeat. Yay.