If it is a programme aim that students should work in groups, then you should think about assessing this skill explicitly. In other words, you may wish to assess the process of arriving at the final outcome as well as the outcome itself.
The University of Melbourne suggests that some or all of the following might be assessed:
You will need to look at the learning outcomes for the unit and for the programme, and at the standard assessment criteria for the programme (if any), to decide how you might assess any of all of the factors on that list within the assignment.
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: