|Nircmd is a good tool....|
As far as I am aware there is no keyboard shortcut for that per-say but there is a series of keyboard shortcuts that can do it(provide you have a win key).
Repeatedly hit tab until the group button is depressed.
<context key> (usually next to the right control key)
If you don't have a context key use <shift>+<F10> instead
Alternatively you could hit the up arrow and enter but that's just more keystrokes.
Of course you may not exactly call that "quick" unless you don't have a mouse/third party utility.