Group boss fights have chat. The bar at the foot of the fight is one line when closed — the newest message — and opens into a short log with an input. Everyone in your instance sees what you send, and you can still talk while dead or during the respawn wait. Messages are kept for safety review, so keep it civil.
Whenever anyone in your instance receives a drop worth over a million coins, the whole instance is told what it was and who got it. It reads in the fight chat in the idle game, and as a purple banner in the open world — where any kill counts, not just a boss.