This is an area where students often feel badly treated with many variations on the theme of “I did all the work and [so and so] did nothing and got the same grade as me” appearing in evaluations of group work.
Once again, the University of Melbourne has a very concise guide to the options available (scroll down to part 4 of the linked section) and it makes essential reading.
Some form of self or peer assessment which contributes to the final mark may help to counter perceived unfairness in a group mark. An example of the implementation of group assessment sheets can be found in Heathfield, M. 1999. These sheets require students to self-assess their contributions to the groups using five indicators: