If you live in the city, the lakefront path can't be beat as long as you stay South of Navy Pier or North of Oak Street beach. The area South of Soldier Field is extremely well maintained and really very beautiful and scenic, it just is underutilized because the neighborhoods on the other side of Lake Shore Drive tend to be rougher as you move further South. That being said, the lakefront path tends to be extremly wel list, busy, safe and well patrolled as long as you're not out after 9pm or so. Other advantages include accurate mile markers, reliable surfaces and plenty of talent to look at (more so on the North side).
With regard to groups, your best bet is one of the racing teams that the various running stores organize. Fleet Feet or Universal Sole have respectable, if not great, teams with some decent post-collegiate runners.
If you're in the burbs, try the Illinois Prairie Path, which runs from Hillside out past Naperville and is a soft, crushed limestone path.
Finally, I would add that the most successful distance program in the country (York) is based outside Chicago and conducts most of its workouts running around a quarter or half mile loop, which just proves that where you run doesn't matter nearly as much as how hard you work.