There are many different ways of forming “effective” groups for students to work in a PBL or PrBL classroom environment – based on their interests, academic rankings, random selections, etc..
Now, the following article talks about some interesting points on the group selection process, be it student-selected or teacher-selected – http://www.facultyfocus.com/articles/teaching-and-learning/group-work-are-student-selected-groups-more-effective/.
This piece of article is basically saying that in terms of the quality of an end product, it does not really matter whether the group is formed by the teacher or by the students. Yes, in an early stage of a class project, a group formed by the students tend to get off the good start as they in general do not need to “ice-break” between the group members.
So, all in all, do not spend too much time on thinking about the ways to form groups. Rather, let that time be spent on the actual group work by the students!