Bunch of different ones. Crankmob and Critical Mass are two big local ones - I believe they both have websites.
For sure, I think they're more fun if you're <21 years old. Even more if you're <18, since it's an excuse to stay out late and hang out with folks slightly older than yourself. They also don't ride much - they go for about 2-3 miles, and then regroup for a hour+ while they socialize. Wayyyy more of a social hangout than a bike ride. Very few 25yr old+ folks there, which makes me think it's more of a thing for HS kids to do at night since they can't go to bars n clubs, but I could be wrong about that.
As a M33 looking for a real group ride, it definitely was the way wrong choice for me . Fortunately, I found the LaGrange cycling club's regular Sunday AM fast racelike Nichols Canyon ride with 40-100+ strong riders, which is truly amazing.