Amass means bring together or assemble. It can be a real shock to enter a room and see your amassed friends shouting "Surprise!"
Although the word amass should not be confused with "a mass," as in the thing you never want to hear has been found on your lung, they both derive from the Latin massa "lump." When you think about it, this makes sense. Whether soldiers or cancer cells, things that come together to form a whole — in this case, a tumor or an army — are amassed.