Shaykhul-Islaam Ibn Taymiyyah (may Allah have mercy upon him) mentioned:
As for the leader of the Hizb, he is the head of the group which has become a Hizb. If they are united upon that which Allah has commanded with as well as His Messenger without any addition or subtraction, then they are believers. For them is that which is for them and upon them is that which is upon them. And if they have indeed added to that or subtracted (from it); like having Ta'assub (fanaticism) for whoever enters into their Hizb (group/party) based upon the truth and falsehood and turning away from the one who has not entered into their Hizb whether he is upon the truth or falsehood, this is from the separation (into groups and parties) which Allah and His Messenger have condemned. For indeed Allah and His Messenger have commanded with the Jamaa'ah and the matter harmony (between the believers) and have forbade the matters of separation and differing. Also Allah and His Messenger have commanded with cooperation upon righteousness and piety and forbade cooperating upon sin and transgression.