외치다 is calling out loudly — a slogan, a team chant, a name. The words being shouted are usually marked with 라고 (반민초! 라고 외치고 있지만, they're shouting 'anti-mint-choc') or run straight in with 하고 (하나 둘 셋 하고 외쳐주실까요, shall we shout it on three?). Elsewhere -고 gives the accompanying act: 이불을 부여잡고 외쳐봅니다 (clutching the duvet and crying out).