There are a group of hadith narrations, not found in al-Bukhārī and Muslim, but found in various other collections, in which the Prophet Muhammad PBUH mentions that the Muslims will divide into 73 groups, 72 of which will enter the Hellfire, meaning that only one among these 73 groups will be saved. This one group is known as al-firqa al-nājiya (“the group that attains salvation”).
These narrations have unfortunately been a favorite polemical tool. Each group can claim to be al-firqa al-nājiya to imply that the members of every other group will enter the Hellfire:
But they tore themselves into sects; each party (self-righteously) happy with what they have.1
The truth of the matter is that these narrations are all likely corrupted or fabricated, and there is no authentic evidence whatsoever for the part that says “all of them will enter the Hellfire except one”.
The Kuwaiti Islamic scholar Dr. Ḥākim al-Muṭayrī (b. 1964, holds PhDs in Islamic studies from Birmingham University and University of al-Qarawiyyin in Fez, Morocco) has conducted a study (Arabic PDF – 3 MB) of all of the relevant hadith narrations regarding this issue. He mentions that Ibn Ḥazm rejected the narration, and that al-Shawkani considered the part that says “all of them are doomed save one” a fabrication. In the conclusion, he writes:
وعلى كل فكل طرق هذا الحديث مناكير وغرائب ضعيفة ومنكرة، وأحسنها حالا حديث أبي هريرة وهو حديث حسن، مع تساهل كبير في تحسينه لتفرد محمد بن عمرو به، وهو صدوق له أوهام خاصة في روايته عن أبي سلمة عن أبي هريرة، ولهذا كان القدماء يتقون حديثه كما قال يحيى بن معين.
All of the ṭuruq (the chains of narrators) of this hadith are objectionable and unauthentic. The best of them is the hadith of Abū Hurayra, which is a ḥasan hadith (i.e. not good enough to be considered authentic, but having an acceptable meaning and not clearly fabricated), provided that we extend it great latitude (i.e. lower our standards) for the fact of Muhammad bin ʿAmr being its only transmitter, who is known to be a truthful person who has awhām (plural of wahm, "confusion" or "delusion", meaning he gets confused and mixes up narrations), especially in his narrations from Abū Salama, from Abū Hurayra, and for this reason the early hadith scholars were cautious of his narrations, as Yaḥyā bin Maʿīn has mentioned.
Note that this best hadith that Dr. Muṭayrī refers to does not have the part that says “all of them will enter the Hellfire save one” (see page 24 of the PDF). The most we can learn from these narrations is that the Muslims will possibly divide into 73 sects (which could possibly be a randomly chosen number used to imply “a great many”, as is typical in Arabian usage).
In conclusion, there is no justification whatsoever for using these narrations to imply that Muslims from other groups will enter the Hellfire; anyone who says such a thing has uttered a falsehood, either out ignorance or dishonesty.