Ruling 1387
Recommended prayers cannot be performed in congregation (in some cases, however, this rule is based on obligatory precaution). [There are some exceptions,] however: ṣalāt al‑istisqāʾ, which is performed to invoke rain, can be performed in congregation. The same applies [i.e. they too can be performed in congregation] to the prayers that were obligatory and have become recommended due to some reason, such as the Eid al-Fiṭr and Eid al-Aḍḥā prayers that were obligatory when the Imam (ʿA) was present and are recommended during his occultation.