The Ruling on Disagreement and Division Among the Groups of Da'wah

Question:
If there is a group on the right calling to Allah , and another group on the left calling to Allah , but the right group insults the left group, and the left group insults the right group, what is the course of action in that case?

Answer:
It is obligatory to advise them so that they unite, and to warn them against division, until they reach an understanding and the problem is resolved. As for this contradiction, it drives people away from the da'wah, until the common person says, "Who is in the right?!" This is fitnah!
Therefore, it is obligatory to unite, cooperate, and remove the disagreement through lawful means. The scholars and people of good should advise them and mediate between them, so that the disagreement is resolved and the da'wah becomes one.
Q: Does one follow one group over another?
A: If he has knowledge, he follows the group that is in the right, and stands with them against the other group until it submits.