Groups of events are mainly for organising events. They can be collapsed and expanded using the + or - button in the title. They are edited with the Event Group dialog. To add an event group, right click an event or empty space in the event sheet and select Add group. Alternatively use the Events tab on the ribbon or press the G keyboard shortcut.
Events can be dragged and dropped in to and out of groups (be sure to drag from the event margin, and not from a condition or action). Entire groups can also be enabled or disabled with the Set Group Active system action, which is useful for situations like pausing the game.