How can I share supporting materials to attendees to a specific event without making it available for all?

There is currently no way to make supporting content only visible to some and not others. A few alternatives may be:

• You can set the visibility of the event to only selected and assigned. This would mean that only those who have access to the event see the content

• You could set an item to ‘Only visible when assigned’ and create a group based on the attendees of the event and set a task for that group