They are very similar, actually, group think refers to the phenom in which the group reinforces its own beliefs to the point that it may become unrealistic in scope. For instance, one of the dangers of group think is to staff your group with people of similar beliefs which do not necessarily reflect the beliefs of the general population. This may lead to something like "New Coke" or a situation where a group of "yes people" reinforce the horrible idea their boss has, creating a delusional representation of society. The Emporer's New Clothes would be an example of this.
Group polarization is a phenom where the group influences or changes the influence of one or more members. A classic example of this is the old study where a volunteer is included in a group of psychologists who can actually influence the perception of the volunteer by unanimously agreeing on something contrary to the perceptive reality of the volunteer. Look into Social Comparative Theory for an example, where a person is concerned about not being considered part of the group or even wrong in their perception and so will change their mind to go along with the group. In one study up to 70% of volunteers who were shown a pair of lines in which one is significantly longer than the other will change their mind or answer if three or four group members before them claim the shorter line is actually longer, and vice versa.
Hope this helps,
Ivery
