A group is especially vulnerable to groupthink when its members are similar in background, when the group is insulated from outside opinions, and when there are no clear rules for decision making. (psysr.org)

Another description, as it applies to education and group projects, is that "Groupthink is a process of gradualism that seeks to gently merge the followers into a pack with leaders, the hope being that the leaders will pull up those who typically reside on the low end of the motivation and achievement scale (sourcewatch.org)

Groups experiencing groupthink do not consider all alternatives and they desire unanimity at the expense of quality decisions. (Small Group Communication)