Never SandBlast, but I've been a pretty regular volunteer at SnoDrift and LSPR here in Michigan.
You typically start the volunteer journey as a marshal, posted up at some random intersection or spectator area to stop people from wandering onto the stage and keep them corralled in a safe area to spectate. Personally, I've always liked being posted at a quiet intersection with a radio operator versus a full-on spectator area. Either way, it's a good place to watch cars drive past and enjoy a day outside (bring a friend). In the video, you can see marshals posted on the left at 0:04 and what looks like a full spectator are at 2:04. Also based on the video, bring some type of face covering. Breathing dust, dirt, and sand all day sucks. I volunteered at Summer SnoDrift once and was blowing dirt out of my sinuses for a week.
Speaking of radio operator: If you're licensed, they're always looking for operators and love when they volunteer. Often, it's local radio clubs that end up supporting the rally. They enjoying being part of a practical application of their hobby.
If you're more interested in the sport, interacting with drivers and co-drivers, and less about watching cars drive past at speed, try to get into timing control. You get to learn about how a rally is timed, scored, and managed and use timing equipment most people don't have the opportunity to experience. Not quite as fun for a first timer who just wants to watch cars go fast.