It took me a few minutes to understand this, but apparently they are in a study trip where they cannot laugh (you know those stupid Japanese comedy variety shows?). I have no idea what Ritsu said (she's probably sleeptalking, I guess), but they mistake Yui and Tsumugi's breathing while sleeping as laughing and announced, "Hirasawa, Kotobuki, out~", prompting an "Eh?" from Yui.