Yeah. When this happens at phase end in the day to day game I give an hour's notice but it was a bit different since it's just pre-game.

If a new person arrives mid-phase, you tend to get the same thing happening. It does mean people miss out on the last few posts but it's always been one of the side-effects.

The idea is that the other group of people won't have heard your conversations and you won't have heard theirs, so it makes no sense to give them that access.