There is not much information I could find on this early Islamic group other than what they believed in and what separated them from other Islamic groups of this time period. If any of you know of any distinctive characteristics such as doctrines or legal practices about them, that would really help me understand what they are all about.
All I have managed to find about the Mughiriyya is a source from the 10th century. It's an Isma'ili heresiography that was written at a time when Isma'ilism was the prominent Shi'a sect (only some years had passed since Abdullah al-Mahdi had founded the Fatimid Caliphate in the Maghrib).
The source is written by an Isma'ili Da'i (a position that served as propagandist, missionary and preacher all at once) known only by his kunya, Abu Tammam, and comes from a larger text. The source itself is a chapter called the Bab al-Shaytan, literally the Door of Satan or Door of the Devil.
In it, he notes that the Mughiriyya seem to worship a man that manifests light, and has a loincloth representing the Qur'an, robes representing the Gospel of Christ, a shirt that has Moses' commandments, etc. They seem to heavily rely on the Mahdi messianic theology, and strongly vilify both Abu Bakr and 'Umar, the first two Rashidun Caliphs.