Alternatively, since people joining at the end of a netsplit tend to already have been in the channel before, you could just make /who not trigger after more than 3 people join within a 2 second interval.

Code:
on *:join:#yourchannel: {
  if (%Join.fld > 3) { return }
  inc -u2 %Join.fld
  who $nick
}


To adjust the time period, change the 2 in -u2, and to change the number of people that can join within that time period before /who doesn't trigger anymore, change the 3 in > 3.

Note that this will still /who the first 3 people, but will stop after that to prevent getting flooded.