Here's how we're doing the 101 sessions this year.
Instead of a whiteboard, I've printed up some blank forms. Anyone who wants to have an impromptu session, grab a form, put the description, location, and time. Anyone who wants to go should sign up. If nobody signs up, then it doesn't happen.
Pro Tip: Post these early, so folks have time to sign up. Last minute sessions won't attract much attention.
Here's an example: