Question if I am obligated to put a mezuza in a shared entrance?

[ChabadKahanist]

 I am living in an apartment building where everyone either rents or owns their apartment I noticed that there is no mezuzah in the front entrance like there was in our old building in Jerusalem. am I obligated to put one up being that it is a shared entrance & being on this side of the building at least one person I know of a Russian who may or may not be Jewish is married to a Goya?