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Click on the group header in the outline panel, and shift click on the handle of the last item in the group, selecting all of the actions in the group. Now press option-= and you'll set the completion percentage of the whole selection to 100%. This works on any collection of one or more tasks. Pressing = instead of option-= will just increase the completion percentage by 10%.

I'm not exactly sure how you're supposed to know this, as it doesn't appear in the menus, the keyboard shortcuts page, or the manual, so far as I can see...