In al-Qur’an, we may find the term 'Rijal-ul-Ghaib' (people of unseen). Who are these people? What are their functions in the existing world? Can we communicate with them? If so, how do we communicate with them in this life?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

The word Rijal al-Ghaib (people of unseen) does not exist in Qur'an.  Probably you may have come across this word in books other than Qur'an.  Ghaib means what is unseen.  Allah Says (interpretation of meaning): {Who believe in the Ghaib……}[2:3] i.e. they believe in what the Prophet (Sallallahu Alaihi wa Sallam) informs them about the unseen, like Paradise, Hellfire, etc. and they do not see them.

Most probably this is a Sufi expression and there are many myths, innovations and polytheism behind this term.

Allah knows best.