A word on the various comments suggesting changing this to apply to shares instead of specified disk groups => DON'T !!
Spin-up groups are very useful if you have controllers that "freeze" activities on attached drives while another drive on that controller is being spun up (this is not uncommon). But if you have, say 10 drives in a "Movies" share, you certainly don't want all 10 of those drives to spin up when you access the share. On the other hand, if a media item you're streaming is on disk x, and you know that any access to disk y will cause disk x to "freeze" until y spins up, you'd most likely want to have x and y in a spin-up group, so when x spun up, so did y ... thus eliminating any hiccups in the streaming.
It's not a critical feature -- this is true of a LOT of the "bells and whistles" that are in the current release -- but it's VERY handy for those of us who use it. It's very easy to test whether it's useful for your particular setup => spin down all drives; start a stream from one drive; and then, one-at-a-time, do an access to other drives and see if any of those accesses cause the streaming video to pause. If so, you've found an interaction you can eliminate with a spin-up group.
Clearly it takes a while to identify all of the interactions -- but once they're all "mapped" you'll never again have a movie freeze because somebody else accessed the server.