I went to a course on active learning and the development of learning groups. One of the things they taught us there and I found it to be really useful is to develop a questionairre and a rating system based upon the topics that you are learning. You then have the students line up based upon their rating with low at one end and high at the other. They then count down in how many groups you want. I have found that this allows for a mixture of students who have knowledge in the area as well as students who has no experience in the area. I would ask questions such as how long have they worked in the field, their age, how many years of college have they taken, etc.