Everyone that rides a trial also observes that trial.  Here’s how it will work:

Riders are divided into 3 groups-A, B, and C at signup.

Within each group, they are also assigned a number from 1 to 8. 

So everyone has a unique designation: A1, B6, etc.  Minimum of 24 riders needed.

After the riders meeting, everyone heads out to the course.

A riders report to their section to observe. A1 goes to section 1, etc.

B riders start their first loop.

C riders start their first loop.

After one loop

B riders report to their section to observe and relieve the A riders

A riders start their first loop

C riders continue and start their second loop.

After two loops

C riders report to their section to observe and relieve the B riders

B riders start their second loop

A riders continue and start their second loop.

Once the third loop is complete, all will have ridden two loops and observed one loop. 

Then A again reports to their section to relive the C riders and the process repeats.

At the end of the day, all will ride 4 loops and observe two.  This should go pretty quickly since only 2/3 of the riders are on the course at any given loop and there is an observer at each section.