Game concept
Activity Host
The game also links it as Activity Host's, Host.
From the game's encyclopedia
The Host of an Activity is the Ruler that started it.
They choose the Activity Type, Location, Activity Options and which Guests to invite.
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
Activity Host is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "The Host of an Activity is the Ruler that started it." It is part of Activity. The game also links it as Activity Host's and Host. Patch 1.20.1.2, from game/common/game_concepts/00_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/00_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.