The people present were Kaladin, Shallan, Dalinar, Szeth, Jasnah, Renarin, Lift, Taln, and Ash. That exactly matches nine of the flashback characters, so I think the person missing was ... Eshonai.
Why would it be that when those nine stand together they instinctively feel both connected and like one person is missing? If it were just about having all ten orders of KR represented, it seems like an awful coincidence that they also all happen to be flashback characters.
And if the missing person really is Eshonai, does that mean there is an in world reason the ten people flashback characters are important?