You're going to need more racers as you progress to the higher events, but I find you can level new drivers really quickly if you start them in Pro sereies events. They won't win, and will spin out everywhere, but they will level pretty quickly since the events are worth a lot of XP.
Also, equip the best tires you can for the event. I try not to spend too much on their cars, but I usually add a few mods (racing filter, ECU, tires if needed, and the first weight reduction stage) and that's usually enough to help them win once they gain a few levels.