Wcsherry asked me to post this.
A group had taken a quest. The party size was 8 (the max on the quest).
Another player encountered them and asked to come along.
He used one of his DM tools to raise the player limit on the quest from 8 to 10 players.
They invited the player, and tried to add him to the quest, but could not.
Long story short, when a DM tweaks the player limit on the quest, the tweak only works if the quest has not already been taken.