Is it permissible to perform supererogatory prayers (Nawāfil) in congregation, such as Qiyām al-Layl (night vigil prayer) or Ṣalāt al-Ḍuḥā (forenoon prayer)?

There is no harm in performing supererogatory prayers (Nafl) in congregation occasionally, but not continuously.

Shaykh al-Islam Ibn Taymiyyah said: "Voluntary prayers in congregation are of two types: First, those for which regular congregational prayer is prescribed, such as the eclipse prayer (Salat al-Kusuf), the prayer for rain (Salat al-Istisqa), and the night prayers of Ramadan (Qiyam Ramadan). These are always performed in congregation. Second, those for which regular congregational prayer is not prescribed, such as night prayers (Qiyam al-Layl), regular Sunnah prayers (Sunan al-Rawatib), the forenoon prayer (Salat al-Duha), and the prayer of greeting the mosque (Tahiyyat al-Masjid). If these are performed in congregation occasionally, it is permissible. However, a regular congregation for these is not prescribed; rather, it is a disliked innovation. The Prophet (peace be upon him) usually prayed his voluntary prayers alone, only occasionally praying in congregation with Ibn Abbas, Hudhayfah, Ibn Mas'ud, and with Anas, his mother, and the orphan."

Shaykh Ibn Uthaymeen was asked about the ruling on performing supererogatory prayers in congregation, such as the Duha prayer. He replied that there is no harm in it occasionally, citing the action of the Prophet (peace be upon him) who prayed in congregation with some of his companions, such as Abdullah ibn Abbas, Abdullah ibn Mas'ud, and Hudhayfah ibn al-Yaman. He concluded by saying that there is no harm in performing some supererogatory prayers in congregation, but it should not become a regular Sunnah such that every time they pray the Sunnah, they pray it in congregation, because this is not prescribed.