Dominaria Spoiler Digest - Who's Who and What's What from the Release Notes
Banding (Any creatures with banding, and up to one without, can attack in a band. Bands are blocked as a group. If any creatures with banding you control are blocking or being blocked by a creature, you divide that creature's combat damage, not its controller, among any of the creatures it's being blocked by or is blocking.)
10/1/2008 If a creature with banding attacks, it can team up with any number of other attacking creatures with banding (and up to one nonbanding creature) and attack as a unit called a “band.” The band can be blocked by any creature that could block a single creature in the band. Blocking any creature in a band blocks the entire band. If a creature with banding is blocked, the attacking player chooses how the blockers’ damage is assigned.
10/1/2008 A maximum of one nonbanding creature can join an attacking band no matter how many creatures with banding are in it.
10/1/2008 Creatures in the same band must all attack the same player or planeswalker.
10/1/2009 If a creature in combat has banding, its controller assigns damage for creatures blocking or blocked by it. That player can ignore the damage assignment order when making this assignment.