Typically opens January 1. While it doesn't sellout in a day or anything, they're usually closed up within a month or so. One thing that I believe is true is they do allow for switching classes post entry, so the main thing is getting money / registration in to reserve a spot at the event.
I've been (borderline) infatuated with this event since I heard of it. I've tried to rally my dad to do it with me as a family duo event on our old Yamaha IT's - he's 61 and has a desire to ride the 50+ ironman (solo) on his '81 IT465! A team of all Alta's would definitely be fun and have the edge on a dead engine start - at this event, first rider of each class actually runs (~20 feet) to bike and then takes off. So long as conditions were OK, and the gearing was right, think you may be able to eek out 2 laps per rider (on a team of 6) before needing to pit. Fun part is, you'd be able to have a remote speaker playing pump of music or something during those lonely hours of the night haha.