Follow all the guidance on the Undergraduate Group Assignment page. It applies to Postgraduate assignments also.
Group Assignments for the Postgraduate Scale
In the ‘Grade’ section of your assignment settings, set ‘Type’ to ‘Point’, and ‘Maximum Grade’ to ‘100’.
In your gradebook, adjust as shown in the screenshot below. The ‘Grade Display’ can be set to ‘Letter’ or ‘Letter (Real)’, the latter also showing the mark out of one hundred.
A learning technologist will be happy to discuss all aspects of your group assignment and check it for you before it goes live. Contact email@example.com