Is supplicating in a collective manner impermissible and an innovation?

It is permissible to gather for supplication (Du'a), remembrance (Dhikr), reciting the Quran, voluntary prayers (Nawafil Mutlaqah), and similar acts, as long as it does not become a regular habit at specific times. If it becomes so, then it is an innovated Bid'ah, as mentioned by Imam Ahmad and Ishaq ibn Rahawayh.