This semester I waited a bit longer to form groups. I also considered their academic performance as of the first exam, when determining groups.
Students who were in danger of failing the class weren't allowed to be in a group -- they had to complete a paper to pass. This is a good thing, as most of those folks have since faded away from the class.
In one section I made a group out of several folks who weren't doing well, but really wanted to do a presentation. That group is shrinking pretty quickly -- and none of the remaining members made it to class last week... sigh. I kind of wish I hadn't let them be a group, because one less group would mean that each group would get a full class period to present.
Overall, the groups seem to be functioning much better than they did in previous semesters. I'll know more when I meet with each group on Tuesday.
Today I'm arranging my T/Th logic class into small working groups -- based on their most recent quiz scores -- I think this is the right time to do so, as they will really need some logic-pals to get a good grade on the last quiz...