O Conselho de Elrond era um conselho secreto?

Sim

Embora muitos dos participantes tenham aparecido em Valfenda por acaso, o acesso ao próprio Concílio foi apenas por convite.

Frodo e Bilbo foram convidados, mas Sam não foi (ênfase minha):

Suddenly as they were talking a single clear bell rang out. 'That is the warning bell for the Council of Elrond,' cried Gandalf. 'Come along now! Both you [Frodo] and Bilbo are wanted.'

Frodo and Bilbo followed the wizard quickly along the winding path back to the house; behind them, uninvited and for the moment forgotten, trotted Sam.

^{Fellowship of the Ring Book II Chapter 2: "The Council of Elrond"}

E Elrond depois chama o conselho de "secreto" quando admoesta Sam por escutar, embora ele também não pareça muito chateado com isso:

'But you won't send [Frodo] off alone surely, Master?' cried Sam, unable to contain himself any longer, and jumping up from the corner where he had been quietly sitting on the floor.

'No indeed!' said Elrond, turning towards him with a smile. 'You at least shall go with him. It is hardly possible to separate you from him, even when he is summoned to a secret council and you are not.'

^{Fellowship of the Ring Book II Chapter 2: "The Council of Elrond"}

Mais tarde, Pippin está bastante chateado com o tratamento da luz de Sam, sugerindo ainda mais a natureza secreta do conselho:

Later that day the hobbits held a meeting of their own in Bilbo's room. Merry and Pippin were indignant when they heard that Sam had crept into the Council, and had been chosen as Frodo's companion.

'It's most unfair,' said Pippin. 'Instead of throwing him out, and clapping him in chains, Elrond goes and rewards him for his cheek!'

^{Fellowship of the Ring Book II Chapter 3: "The Ring Goes South"}

Apenas para adicionar à excelente resposta acima, foi um conselho WAR , e por isso necessariamente secreto de olhos curiosos.

The process of the council is significantly shortened. There are no or minimal references to the history of the Second Age, the Rings of Power, the current War, the politics of the races and the characters' backgrounds. The only characters who speak are the protagonists who will later form the Company of the Ring.^source