I've just noticed this so I have no idea if they are fully booked or not.
"Skinny Tyres have teamed up with the organisers of the epic Scottish sportive, the Etape Caledonia, IMG. We are delighted to be able to offer pre Etape Caledonia training rides on February 6th, March 20th and April 10th. These events will be single day rides, but supported with guides. The rides will be free of charge, but must be booked through this website and places will be limited to 30 riders. Places are now open for booking."