Hey, WAG, good to see you on the board once more. I've missed you. Or have I overlooked you? Either or....
I don't do group work as much as I used to. The problem you've cited is one reason why. It is THE reason why I insist my students do their own, individual, science prpjects for our science fair.
When I do group work, I do the following:
** I permit students to chose their own groups. Yes, I know this opens up to the potential that students will do more socializing than work, but it also exerts peer pressure, where one does not dare let the group down lest there be reprecussions later on.
** I insist that each member of the group take responsibility (in writing) for a certain aspect of the project work, ie graphics, writing the narrative, researching information.... Each of those aspects is then graded and if one particular thing is not done well, the one in charge of the aspect is penalized, not the whole group.
** I sometimes have students do a sort of "after action" report (my military background shows here) in which they critique the job they as a group did, mention anything that they did well or couild do better and do an self-evaluation as well as an evaluation of each group member. All of this is factored into the final grade.