Select your table and sort the column where the groups are displayed. To sort, go under Home and Editing section. Click on „Sort & Filter” and choose on of the first two. Next go to Data and click on Subtotal. Under „At each change in” select the column the group is in. In the next section select the function you want to be used and in the third bit select the column the subtotal is added to. Click on OK when you’re done.