I had an interesting experience in teaching in forming a number of groups in a class.
My experience is that you need to really understand the class and individuals prior to allocating groups.
I noticed some of the students did not want to be in particular groups as there appeared to be some issues in doing so. This was confirmed when I continued to organise the groups under protest from the students in some areas.
I believe it is absolutely necessary to understand the students and to ensure you have a relationship with them prior to organising groups.