Creating A Group Assignment
You'll want to make sure your groups have been created first and everyone has been enrolled in them. You can find a tutorial on creating groups HERE.
To create an assignment for a specific group, you will go to the Assignments Tool in the Navbar, then select New Assignment.
When the assignment page opens up, you build the assignment as you would any other, but on the right side, in the section titled Submission and Completion, choose GROUP assignment instead of individual assignment, under assignment type. Then a drop down menu will appear, where you can choose the group you want to add. Be sure to click Save and Close at the bottom of the page.