Game concept
Special Guest
The game also links it as Special Guests.
From the game's encyclopedia
Special Guests hold a special place of honor at Grand Activities. Honor someone at your Feast or select a Champion to represent you at your Grand Tournament!
They are more likely to accept your invitation and any activity to which they are invited will not begin until they arrive.
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.
Special Guest is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Special Guests hold a special place of honor at Grand Activities." It is part of Activity. The game also links it as Special Guests. Patch 1.20.1.2, from game/common/game_concepts/04_ep2_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/04_ep2_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.