One of the news articles I saw listed the Sheriff's office phone number, so I called that and they said volunteers were meeting up at the community center. I drove out there, they had a check in sheet at the entrance. I got there around 9 or 9:30, they already had the 10AM teams organized. It looked like most everyone there was in uniform, SAR teams from different areas and so on, but there were a few volunteers waiting around too. I think there were a lot more volunteers already in the search area.
Once they were done with their briefing I asked around what I could help with, I was introduced to the IC (incident commander?), he asked if I knew the area, I told him I didn't know anything east and south of Rabbithole so I'd be more useful as another set of eyes, and he told me to hang around. The next set of teams would have gone out at 2PM, but the family was located around noon. Very cool watching the good news quietly spread. As soon as the press started to swarm I signed out and left, probably a few minutes before they brought the family in.
This was the first time I had walked in to a situation like this, if I did it over again I'd immediately start asking who to talk to to be useful sooner. Everyone I talked to made me feel welcome, but walking in on day 3 I think they assumed everyone there already knew what to do.
The SAR teams periodically stop their engines, get out, whistle, and listen. One news report said the family heard one of the teams whistles nearby the day before, but weren't able to get noticed. I never thought about keeping a whistle in my glovebox, though I usually have one in my daypack. Or maybe throw a canned air horn in the kit.