For quite a few years now, in order to regulate and reduce the vehicle traffic flow to Maroon Lake, the Forest Service has restricted vehicle traffic to Maroon Lake. Go to this link to search for detailed information: fs.usda.gov/whiteriver, but generally speaking, you can only drive a vehicle in before 9:00 AM or after 5:00 PM. Limited parking is available and can fill up rapidly on summer days. If arriving between 9:00 AM and 5:00 PM, you'll be required to ride a shuttle bus that runs every 20 minutes up to the lake.
From State HWY 82 on the west side of Aspen, drive through the traffic circle south heading for the Aspen Highlands Ski area and Maroon Lake. This traffic circle is about 1 mile west of Aspen on HWY82 or 40 miles SE from Glenwood Springs, and after the airport and Aspen Business Center by the airport. The road number is CR13. The so-called "welcome station" is 4.7 miles south and that's where you must pay a vehicle use fee of $10. If attempting to drive in during restricted hours, you'll have to park at the Highlands Ski area and ride a free shuttle in. The trail to Crater Lake is what you need to locate at the SW end of the parking lot. The trail goes around Maroon Lake on the north and west side.
Designated, fee campsites are available at the Silver Bar CG, the Silver Bell CG and the Silver Queen CG along CR13 as you drive in to Maroon Lake. These campgrounds are almost always occupied, especially on weekends. The White River National Forest website indicates that sites in these campgrounds can be reserved by calling 1-877-444-6777. There is no at-large camping allowed anywhere else along the road to Maroon Lake. Technically, vehicle camping in the parking lot at Maroon Lake is also off limits, but still practiced by many. Attempt at your own risk. The nearest other campground will be the Difficult Creek CG up HWY 82, about 4 miles east of town toward Independence Pass.