Is it permissible to sit with people of innovation - especially the Sufis who have become numerous in the land and have fought against people of sound doctrine - out of a necessity for da'wah?

The general rule is that it is impermissible to sit with people of innovation (Ahl al-Bid'ah), as doing so implies an endorsement of their falsehood and increases their numbers. However, if a legitimate Sharia benefit arises from it, such as refuting them or exposing their falsehood, then it is permissible. This is specifically for those whose knowledge is firmly established and who possess the ability to refute and dismantle specious arguments, similar to the actions of Ibn Abbas and Ibn Taymiyyah, and is not permissible for the general public. As for without any benefit, one should not listen to them nor sit with them, in accordance with the Almighty's saying: "And when you see those who engage in [offensive] discourse concerning Our verses, then turn away from them until they enter into another conversation. And if Satan should cause you to forget, then do not remain after the recollection with the wrongdoing people." (Quran 6:68). Al-Hasan said: "Do not sit with the people of desires (Ahl al-Ahwa), nor argue with them, nor listen to them."