Guru Nanak Dev founded the Sikh religion. He was followed by nine other Gurus of Sikhs and a holy book 'Guru Granth Sahib' which was edited by the 5th Guru of Sikhs, Guru Arjun Dev leaving the verses to be added by 9th Guru of the Sikhs, Guru Tegh Bahadur.
On his death bed, the 10th Guru of the Sikhs, Guru Gobind Singh stated that henceforth this holy book would be the Guru of the Sikhs. Now, the Gurus have said, "Dhur Ki Baani Ayee Jin Sagli Chint Mitai" (This holy book is the voice of God Himself which has removed all the worries of Mankind).
Anybody believing in a Guru other than the Ten Sikh Gurus and the Holy Guru Granth Sahib is not a Sikh. Hence, Namdharis, Nirankaris, Ravidassias, followers of various Deras, etc. are not considered Sikhs. I am also not a Sikh because I do not believe in everything that is written in Guru Granth Sahib.
Similarly, Ahmadis like me can’t be considered as Muslims as they do not believe in the finality of Prophet Muhammad while I believe in the incarnation of God. In fact, they have been declared as non-Muslims in Pakistan and are persecuted in Muslim majority countries like Bangladesh, Indonesia, etc.