Most of my riding is solo. Commuting is solo due to it's nature. I've also been on 40-60 miles rides completely solo. It's really nice when you're in no hurry, and want to take pictures.
However, I recently helped form a friendly no-drop group ride. We ride once a month in any weather. We try to make it interesting with stops for food, coffee, or beer. The rides are usually 25-35 miles, but I just led one this month that was almost 50 miles, and really hilly. It was still fun, but we didn't stay together. Our group is
River City Cycling Society.
The "big" group in the area is the Louisville Bicycle Club. I'm not a member, and have never been on one of their rides, but I will probably check out some of their rides soon.