I'd like to start by saying that I am a swing manager at McDonald's and have worked in about 4 or 5 different stores. I have never heard of anyone getting a group interview. However I can probably help you out with the type of questions they will ask, as they will most likely be the same as the ones asked in a one on one interview. McDonald's hires many teens, and for most of them, it is a first job. They will not ask much about work experience or anything like that. Most questions are gauged toward customer service, such as "If a customer came up to you, and said that (s)he had received cold fries with her meal, what would you do?" Other questions they will ask will be personality questions, because McDonald's is all about the teamwork, and they want people who will fit into that team. Just remember, they are not expecting a formal or professional interview. They just want you to be yourself. Don't sweat it, it's really not that difficult. Hope this helps.
They will most likely make you do ice breaker activities and see how well you work with others. Sounds easy to me and much easier than having the spotlight on you. Don't worry too much.