Welcome to United Bimmer! Was the coolant level low? Since you added coolant you need to bleed the air out of the system. When air gets into the system it prevents the coolant from circulating which is a leading cause of overheating. I'm not to familiar with the 7 Series but their should be a bleed screw next to the expansion tank. You'll need to have the heat controls on full hot and run the engine to get it to normal operating temp. Periodically opening the bleed screw to let air out. If you see little air bubble it means their was air in the system. Go until you only see coolant their. It may be benificial to lift the nose of the car a few inches to help the air rise to the top.
Also check for coolant leaks under the car and at all hose connections. The coolant hoses wear out over time. Let us know of this helps.