This article explains what settings you need to change when creating a Turnitin Assignment for a group.
How do I create a Turnitin assignments for groups?
To create a Turnitin assignment for Group Assignments follow the steps of How do I create a Turnitin Assignment?
However, you just need to change 1 setting, to make the assignment a group assignments, instead of an individual assignment.
Under Assignment Type, select Group Assignment and choose the desired Group Category.
How do I assess the Turnitin Group Assignment?
You can review the Turnitin group assignment just as an individual Turnitin Assignment.
The only difference is that when you grade a particular submission, all group members will receive the same grade and the same feedback. You are not able to score individuals within a group assignment.