Using a crock pot to carry grilled hamburgers

I would cook them normal to get the singe and just very slightly under done if any and put them in the crock pot. Cover them with a warm mix of one half water and one half regular Coke. Warm them to steaming when you get there.  I know it sounds strange, but many catering services use this method to keep them hot and "fresh" a couple hours or so.  It works, your guests will think they just came off the grill.

Good luck to you.  .

There is no such thing as holding and transporting Hamburgers and keeping them Medium without some professional and expensive holding equipment. Unfortunately to keep them safe they need to be 140+ that is already medium. Crock Pot Warm settings are designed to keep food safe and cover the company's A$$. They typically run 150 to 165, Well Done. If you try to put undercooked Ground Beef at these temps and let that finish the cooking, at a minimum you still end up with well done burgers and at worst some burgers spend a couple hours in the Danger Zone and some older family members and young kids end up sick. Not a fun Party! 

I have never heard of the Water/Coke thing at any of the catering companies I worked for, but that was 15 years ago and new tricks may have become popular. We always used Beef Broth with success. The burgers were cooked to medium, IT of 140°F, then singled and held in 2" pans to keep all evenly hot. The hot burgers were half covered with hot broth, covered, and placed in holding cabinets at 150° or chilled and reheated to 165°F. Either way there was No Pink, essentially well done, but not dry and tough either.

At home or places where a grill is available, I will cook and serve med/rare or medium burgers. But I would not play around with transporting Rare burgers for 2 hours and hope they are cooked and safe in a Crock Pot...JJ