From e-Learning Documentation
How do I create a student group assignment?
You can create a group assignment by using the Group Assignment checkbox. Canvas uses Group Sets to assign group assignments, and each Group within the Group Set will be required to complete the assignment.
Click the Assignment link.
Create a New Assignment
Click the Add Assignment button to create a new assignment.
Assign Group Work
Click the This is a Group Assignment checkbox to assign group work.
Select Group Set Drop-down Menu
Click the Group Set drop-down menu to select which groups will be assigned  or create a new group category . Note: Self sign-up groups cannot be used for Group Assignments and do not appear in the drop-down menu.
Assign Grades Individually
You can choose to assign grades to students individually by selecting the Assign grades to each student individually checkbox. If this box is left unchecked, all the students in the group will receive the same grade.
Note: In group assignments, one submission will count for the entire group.
Click the Update Assignment button to update the assignment.
Click the Publish button to make it visible to students.
View Published Assignment
View published assignment.