Terms get misused all the time, and I too, am guilty of such. However, a "Band" is any number of musicians that play together, whether they sing or not is irrelevant. A "Group" is just any number of people, and/or musicians that are together for anything....(fill in here, if you deem nessesary.). A "Group", referring to a "Band" is just quick slang. It does have correctness though: The Supremes were not a "Band"! "Vocal Group" would clarify duos, to any number, unless it's a crowd, then "Choir" should be employed. Duo, Trio, Quartet, Quintet,....or just a plain "Mob",...that may start dancing. If there are paid to do so, a "Troup".
........Have a good day, and I thank you....for learning!