You can select tasks in the dashboard several ways:
One by one:
Mac: CMD + Mouse Left Click
Win: CTL + Mouse Left Click
In a range:
Mac/Win: SHIFT + Mouse Left Click
Within a map region:
Mac/Win: SHIFT + Mouse Left Click and Drag
In addition, you can Select All tasks using the CMD + A (CTL + A on Windows) shortcut on your keyboard.
Much like Zoom-to-fit and right click options, the select all function is contextually aware. That is, it will select all tasks based on the current list you are using. For example, clicking a driver and using the command will select all tasks in a driver's queue. Pressing it again will move up one step and select all tasks in that driver's team. Pressing it a third time will select all tasks currently visible in the dashboard.
Pressing CMD + A (CTL + A) again adds tasks in the team to the selection: