One of the things that I tell students and any group to watch out for is the problem of group-thinking. Group-thinking occurs when a group puts when the goal of acting or making a decision becomes more important than making the right decision. The right decision requires open discourse of the topic and often times requires taking a careful look at opposing view points.
Disagreeing just to disagree is not always recommended; however, a good decision usually requires looking at the issues from multiple angles.
The challenge of using students groups is keeping the group on task. A way to overcome this challenge is establish due dates of group task. This allows me to work with groups so they can successfully complete a given task. It also gives me an opportunity to identify who in the group may need additional help and clarity.