I think the better question is not "who" is the 7th member, but rather "why" are they not present at that critical juncture... the rest of the members are there.
IMO there isn't a 7th member, that 7th seat was Alexander's and he was dead set on Kane being "tested", so I think the 7th seat is being reserved until Kane proves to be worthy or not, in which case the seat would go to someone else, probably Olivia, maybe even Seth... that would explain why that 7th member is MIA during such an important transition period, because otherwise it makes no sense.
What could possibly be important enough for a 7th member to be away and not directly involved in the power struggle that is happening at the moment? Sometimes the most obvious answer is the right one: there is no 7th member of the board (yet).