Is the utterance of the worshiper, "Amen, we bear witness, glory be to You, and so on," during the Qunut supplication legally sanctioned, and if so, when should it be said?

If the Imam performs Qunoot in prayer, the congregants say "Ameen" to his supplication. However, when the Imam praises Allah, it is better for them to remain silent, suffice with his praise. There is no basis for the congregants to say "Nash-had" (we bear witness) or "Ya Allah" (O Allah), or similar phrases during the Imam's Qunoot.