We made the fatal mistake of not booking ahead which is a prerequsitie After driving from Missoula, we saw a sign about the tour but, on arrival, were told that no booking meant no tour. We were told there "may" be a spot for the next one about three hours later - if there were cancellations!.
We simply weren't prepared to wait on the off-chance that we would get a spot. It seems odd that provision isn't made for passing tourist trade but I guess that has to do with safety requirements and volunteer rosters. We did. however, have a wander around Pendleton and enjoyed checking out the historic architecture and some of the shops.