bhubenig are you shift-clicking the create metaball icon? Metaball booleaning doesn't work like normal boolean. You shift-click the metaball icon and it will create a metaball that looks just like a normal one (good idea to rename them so you can find them again). Then when it comes near a positive metaball it will subtract when rendered. No grouping needed. For more on metaballs search the forum as a lot of things have been explained before. Hope this helps