When creating a custom dashboard, you have the option to select all subgroups for your search. You can do this by clicking or putting your mouse over the arrow by the "Filter & Display Subgroups" box.
This displays the full range of subgroups for which there is data on your site. To quickly select all, just click.
At any point you can click the little "x" on the left side of an individual subgroup to remove it from the filter.