a BAND is where members play intruments and typically one person, or sometimes more, do the vocals. a GROUP, as in a group of singers who do the same job, though many groups also have lead singers.
I think he's specifically referring to vocal groups.
That being said, the omission of The Temptations, The Supremes, En Vogue and Boyz II Men from the poll is absolutely inexcusable. Seriously, Backstreet Boys but no Boyz II Men?