We were in the same situation last year May 31st and June1st and decided, based on advice here, to reserve out seats on the earliest shuttle as soon as we could, in late Jan I think. I talked by phone with the reservation mgr who faxed me the schedule for each day. We faxed our request to him, he faxed back our confirmation for 8:00am the 1st day and & 7:30am the second day. When we got there, we found out they had added earlier buses on both days. Our buses were fairly crowded, not every seat and not one per seat. Reaching over people or vis versa was necessary to try to get photos, especially when we had a grizzily right next to the bus. You can book your bus, and can later change the time or cancel up to two hours before each tour. You can probably get on without but might have to wait for the next bus or two. A schedule is available on the denali website under transportation, shuttle bus.