If you wish to see activity log for all contacts in a certain sub-unit, you can click on the "Statistics" tab.
In the drop-down menu, select "Activity log per unit" in the top left corner. You should now see a window with your organisations structure. Select the appropriate unit by double-clicking on it’s name. You should now see the activity log for all contacts in the sub-unit. Tip! Use the filter menu to filter the results.