If students have not already submitted work, then go to the assignment and choose to Edit it. Go down to the settings near the bottom and there is an option to make it a Group Assignment. You need to have groups set up before they turn work in or any work submitted will be submitted as individuals. You can make it a group assignment then choose to set up groups later or even create the groups at that point.
If students have already submitted work, the system will not let you convert it to a Group Assignment. You will need to make a duplicate assignment, with the groups option turned on, then publish and direct students to that assignment.
