Possibly Riderbee, since IIRC they are similar to sidecar and lyft. Zimride and Ridejoy operate like a traditional bulletin board so they should be fine.

When you're dispatching drivers in real time, just like a taxi dispatcher, the law can interpret you as one. The passenger is entrusting the service to assure the driver is safe. That's not a responsibility that the government wants to bestow upon a non-registered entity.