Ref. No. 41/953
In the name of Allah the most Gracious the most Merciful
The answer to your question is as follows:
It is not allowed to offer voluntary prayers like tahajjud prayers in congregation. Some Ulama consider it Mahrooh only if it is done with Tadaee (pre-announcement) while others consider it makrooh in both cases. The Prophet of Allah (saws) and his companions were not accustomed to offering voluntary prayers in congregation. So in general voluntary prayers should be offered alone. This is the view of majority of the Ulama. An individual practice cannot be an evidence for others. One is obliged to follow the accepted norms of believers.
And Allah knows best
Darul Uloom Waqf Deoband