It doesn't seem fair to punish the whole group when one member doesn't do their share. I've seen a group-assessment rubric, where each member of the group scores not only themselves, but their group members. This is where the guilty one gets leaked out. :) There is inevitably one in every class, but shouldn't bring the whole group down.

On the other hand, my nephew, when he was in 6th grade, was assigned a partner that was a huge slacker, so my nephew did the project himself (after several attempts to do the project with his partner). As the two were presenting it together, it became apparent that it was a one-sided project, b/c his partner couldn't answer any of the questions, etc. The teacher severely docked their grade because it was a one-way project, and nothing was done to let the teacher know of the problem, etc. It was a life lesson that my nephew will never forget, but one that taught him to speak up when there is a problem, and to let the "boss" know of issues that he/she should be aware of.