Iran is known as a land of cultural diversities and for thousands of years numerous tribes and communities lived in this land. They have migrated from other parts of the world and have found Iran as a peaceful homeland and have considered it as their own.
This news report by Press TV on Sikhs living in Tehran.
Gurdwara Sahib (Tehran) was founded in 1941 by Bhai Ganga Singh Sabha Tehran. Although Sikhs have been living there over a century. Religious celebrations include morning and evening prayers, Guru-Ka-Langer every Friday after the Akhand Path. Community services include establishment of a school, teaching of Punjabi and Dharmik (Divinity) forms an integral part of the curriculum. There are about 800 members of the Sikh community in the area.
Here is an interesting Conspiracy Theory on Sikh-Iranian link:
Similarity between the “Khalsa” symbol and Iranian flag, which is supposed to be a calligraphic form of the word “Allah”. Now, this symbol was picked by Ayatollah Khomeini. Khomeini’s grandfather was an immigrant from India, and had the surname “Hindi”. So there is always a claim made by Khomeini’s enemies that his grandfather was a Sikh and that he too was a Sikh. And the similarities between the two symbols are supposed to be the proof.