I recommend that you go to this website:
http://steamspecials.com/ and get yourself on the mailing list for Historic Transport Preservation (HTP). Not only do they run charters like this, but they occasionally advertise other private and publicly-announced charters to their client list. As Michael pointed out, Willow Creek Productions ran a number of private charters over the years, none of which were announced publicly. The operator typically invited the folks who had participated in previous events and if he needed to fill some slots, he asked his clients to recommend friends. That's how I got on the list. In the case of the event two weeks ago, they did need a few extra folks to make it viable, and HTP did advertise the event to their clients. I presume that was how the slots were filled.
Unfortunately, as Michael also pointed out, the operator did inform us all that this would be his last rodeo. On the good side, there are a number of other organizations that run charters, and of late, there has been no shortage of events for folks who like to photograph steam.
/Kevin Madore