add_to_entourage_court_and_activity_effect

add_to_entourage_court_and_activity_effect = {
	#If the character is a pool character
	$NEW_COURT_OWNER$ = {
		if = {
			limit = {
				#Pool characters get automatically added as courtiers
				$CHAR_TO_ADD$ = { is_pool_character = yes }
			}
			show_as_tooltip = { add_courtier = $CHAR_TO_ADD$ } #This already happens with add_companion
			hidden_effect = { # This must go above activity effect, or they will try and make their own travel plan!
				if = {
					limit = { $CHAR_TO_ADD$ = { NOT = { exists = current_travel_plan } } }
					$NEW_COURT_OWNER$.current_travel_plan ?= { add_companion = $CHAR_TO_ADD$ }
				}
			}
			if = {
				limit = { exists = involved_activity }
				$CHAR_TO_ADD$ = { add_to_activity = $NEW_COURT_OWNER$.involved_activity }
			}
		}
		#If the character is NOT a pool character
		else = {
			if = {
				limit = {
					$CHAR_TO_ADD$ = {
						NOT = { is_courtier_of = $NEW_COURT_OWNER$ }
					}
				}
				add_courtier = $CHAR_TO_ADD$
			}
			hidden_effect = { # This must go above activity effect, or they will try and make their own travel plan!
				if = {
					limit = { $CHAR_TO_ADD$ = { NOT = { exists = current_travel_plan } } }
					$NEW_COURT_OWNER$.current_travel_plan ?= { add_companion = $CHAR_TO_ADD$ }
				}
			}
			if = {
				limit = {
					exists = involved_activity
					$CHAR_TO_ADD$ = { can_join_activity = $NEW_COURT_OWNER$.involved_activity }
				}
				$CHAR_TO_ADD$ = { add_to_activity = $NEW_COURT_OWNER$.involved_activity }
			}
		}
	}
}

As written in game/common/scripted_effects/00_activity_effects.txt.