During a carpool, the driver is called the carpooler but I can't find any words for the passengers who share the same car, how can I call them?
To me, and apparently to the compiler of Collins dictionary, a "carpooler" would be a member of a carpool, whether they drive or not. Many carpools are structured so that different people drive on different days.
If a carpool has a single driver you can just say "the driver in the carpool" or "the nominated driver of the carpool". Another phrase is "designated driver", which is often used when a group of people go out drinking, but one chooses to stay sober and drive the others home. Similarly you could say "passengers in the carpool" or "non-drivers of the carpool".
I think you could use either of these:
Of those two, I think passenger sounds more formal, and rider more informal.
It’s easy to find dictionary definitions supporting passenger, but finding one for rider was harder to do. I did find one on the Wordnik page:
rider (noun) a traveler riding in a vehicle (a boat or bus or car or plane or train etc) who is not operating it