Ya, I asked Verlyrus to make it more clear (the one who makes the Design Notes), but haven't heard back:
At 200 CHA, Pets and Guests will do +20 damage per hit, and +20% Exp/Gold earned per Guest.
Until you have 100 CHA, Guests will reduce the amount of Gold and Exp you earn in battle. Above 100 CHA, however, Guests will increase the amount of Gold and Exp you earn in battle!
..as per the Sakurai's own -0.1/+0.2 factor explanation.
For each guest you bring, enemies will gain 60% more damage.
It'd just be more convenient if the game itself'd tell you this generally ("guests increase enemy damage, and reduce Gold/XP earned without sufficient CHA" or w/e) without having to figure things out manually or looking up some in-game mechanics.