All you have to do is hold down the "Shift" button on your keyboard (or Mac equivalent), and click on the metaball icon you normally do to create one. Viola! Negative metaball. No grouping required. Note/warning, if you group just negative metaballs together, they just dissapear from your render, not negative or positive. They're a bit of a pain at first but after say, the 20th curse word...it starts getting fun...

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