Wow. Out of curiosity, are they in person or online? Weekly, bi weekly, monthly? Most people have trouble managing schedules with even one or two groups.
Now, related to going from 1-20. It takes time. Leveling after every session is, imho, to fast. You get stuff to fast, don't have time to try new stuff in game etc. We used to do leveling after every 3 sessions. It gives you enough time to see your character in action at every level, try new powers, new combos, and then move on. With that tempo, it's 57 sessions, so just over a year if you play every week. But, if you take into account holidays, vacation days (20-30 days, so 4-6 weeks), random cancelations, more realistic time frame is 1.5-2 years. That's pretty long time. Even leveling from 3-8, so 15 sessions can stretch to 6-8 months of real time, and that's with pretty regular play.