Cooperation is a common Enchantment — Aura card from Ice Age (ICE), collector number 18. Current market price: $0.13, TCGplayer market prices via Scryfall.
Enchant creature Enchanted creature has 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 a player controls are blocking or being blocked by a creature, that player divides that creature's combat damage, not its controller, among any of the creatures it's being blocked by or is blocking.)