The latest you could checkin would be 7 am. It has to be at least 3 hrs before flight departure. The RAC desk opens around 5 am. I would plan on being there as early as you can...the lines can get long. And everyone in your party should be there also..it's the same as checking in at the airport...IDs are needed.
Your DME bus will most likely be picking you up around 7, so plan on being at the RAC desk by 6, get your boarding passes, and then head over to the food area and grab some breakfast. Then, be at the bus stop by 6:45...when they say the bus picks up at 7, they mean it. They will pull out at 7:04!!