The way the club distinguishes spectators vs participants is via the waiver. A spectator has not signed the SCCA waiver a participant has. Now don't be confused by a waiver someon signs at the guard shack on the way into the track, typically that is the waiver required by the track owner. So in that light a non-spectator race would only mean that everyone netering the track has to sign SCCA's waiver, but a membership is not required.
Now, I have a feeling that risk management probably feels that it is difficult to ensure that everyone entering the site ends up signing our waiver so I think most club races are spectator events, unless maybe you can really demonstrate enough control over the site.