pray two rak‘ahs for Allah, and another person makes a similar vow, congregational prayer is not obligatory for them, since it is not one of the five daily prayers.
Nor is it obligatory for voluntary prayers. Thus, if a person wishes to offer a voluntary prayer, he is not required to perform it in congregation, because it is not one of the five daily prayers.
But is it permissible to offer a voluntary prayer in congregation, or should we say that this is an innovation?
The answer is that the matter requires further detail.
Among voluntary prayers are some for which congregational performance is legislated, such as the prayer for rain, the eclipse prayer—if we hold that the eclipse prayer is a sunnah—and the night prayer in Ramadan.
Other voluntary prayers are not prescribed to be performed in congregation, such as the regular sunnah prayers connected to the obligatory prayers and the night prayer outside Ramadan. However, there is no harm in performing them in congregation on occasion.
The evidence for this is that the Messenger of Allah ﷺ would sometimes perform the night prayer in congregation: Ibn Abbas prayed with him (1), Hudhayfah ibn al-Yaman prayed with him (2), and Abdullah ibn Mas‘ud prayed with him (3). At times, he would even perform prayers other than the night prayer in congregation, as when he led Anas, Umm Sulaym, and an orphan who was with