tournament_add_contestant_or_spectator_effect

tournament_add_contestant_or_spectator_effect = {
	save_scope_as = new_arrival
	if = {
		limit = {
			NOR = {
				is_in_guest_subset = { name = contestant }
				is_in_guest_subset = { name = spectator }
			}
		}
		if = {
			limit = { activity_is_valid_tournament_contestant = yes }
			involved_activity = {
				add_to_guest_subset = { name = contestant target = scope:new_arrival }
			}
		}
		else = {
			involved_activity = {
				add_to_guest_subset = { name = spectator target = scope:new_arrival }
			}
		}
	}
}

As written in game/common/scripted_effects/04_dlc_ep2_tournament_effects.txt.