Game concept
Activity Guest
The game also links it as Guest, Activity Guests, Guests, Attending.
From the game's encyclopedia
A Guest at an Activity is any Character invited by the Activity Host.
While they usually cannot affect the Activity as much as their Host, they can still pursue their Intent.
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 Guest is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Guest at an Activity is any Character invited by the Activity Host." It is part of Activity. The game also links it as Guest, Activity Guests, Guests and Attending. 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.