This article discusses the difference in evaluating group assignments and individual assignments as the workflow might differ slightly.
Grading a group assignment
Whereas with individual assignments Instructors can choose whether to apply the grade in the Grade Book or in the Assignment folder, for Group Assignments where a single grade needs to be applied to all group members, it is required to assign the grade in the Group Assignment Folder.
To add a Group Assignment grade, open the submission in the Assignment folder and add a score to the assignment. The grade will be applied to all members of the group (for this particular assignment).