If an opponent attacks with 6 1/1 elf tokens and I block with 6 1/1 elf tokens, while still on the blocking step after blockers are declared, can I use Helm of Chatzuk to give 1 of my blockers banding to take advantage of banding's damage sharing?
If a creature has banding, its controller chooses how combat damage from creatures blocking or blocked by that creature is assigned, rather than any other player (C.R. 702.21j-k). This is true even if it didn't have banding as attackers or blockers were declared, and even if it's attacking but not in a band.
EDIT (Apr. 18): Correctness edit.