Groups allow you to categorize time entries and get the total amount of hours worked against projects, clients, tasks and more in any given time period. Any time entry field can be used as a group and you can also group time entries by week and month.
Furthermore, you can add as many groups as you need. By combining groups you can create nested report structures and get a break down of your time data by different categories. For instance, you can group time entries by users, projects and weeks to get an overview of weekly time spent by your employees against different projects.
To add a new group to your report go to groups > Add group and select the field that you'd like to use. The "A -> Z" and "Z -> A" buttons allow you to sort groups alphabetically in ascending / descending order.
You can also change the groups' position with drag and drop, using the hamburger button displayed on your right hand side.