Look at 2:48 for the action, as Jin snakily sticks her tongue out, while Jisoo makes a kissy face.
But who are they teasing– Mijoo or Sujeong? And did they start it, or was there an uncaught-on-camera provocation from one of them to start it?
It would be like Mijoo the “mood-maker” to have some sort of game going on with them. If Sujeong is the intended recipient then probably Jin and Jisoo are teasing her for some reason. Perhaps those with an encyclopedic, Hangul-based knowledge of all the Lovelyz Diary seasons and so forth could give an exigesis. But, lacking such abstruse knowledge, we will just have to enjoy the moment caught on camera, as well as the entirety of this “For You” performance!