A brother said that after completing Isha, voluntary prayers must be performed alone, not in congregation. Is it allowed to pray Tahajjud or part of it in congregation with others?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

Praying Tahajjud and other voluntary prayers in congregation or individually is permissible. The Prophet did both. Most of the time, he prayed the voluntary prayers individually. But he prayed voluntary prayers in congregation, as he prayed At-Taraweeh prayers in Ramadan with some of his companions for three successive nights. He also prayed a voluntary prayer (as Imam) once with Huthayfah, and another time with Ibn ‘Abbaas, and a third time he prayed two Rak’ahs with Anas Ibn Maalik and an orphan who lived with him along with a female relative in their home.

He also prayed –as Imam –two voluntary Rak’ahs in the house of ‘Utban Ibn Maalik along with some of his companions. All this was reported in authentic Ahadeeth.

But it is known that the Prophet didn't call the companions to gather as a congregation that is organized and arranged beforehand for a voluntary prayer. Therefore, it might be that the preponderant opinion is that it is better for a person to pray voluntary prayers individually unless he fears getting lazy or that he has a poor memory and might make mistakes while reciting the Quran.

Allah knows best.