Do we, who live in an apt. building, require eruv chazeirot/shituf mevo’ot?
If you live in an area that has a community wide eruv, this would mean that whoever made the eruv, i.e. the tzuras hapesachs around the community, also arranged for a communal shituf mavo’os. Without this, the tzuros hapesach would not help much, as it would still be forbidden to carry from private domains in to the community streets. If you rely on the validity of this “eruv” [tzuras hapesach] and shituf [which also includes sechiras rshus for non Jewish residents], it permits carrying throughout area, to and from all houses, courtyards and streets.
If you live in an area without a community wide eruv and hence carrying is prohibited out into the streets, as this is a public domain, either Biblically [Reshut Harabim] or Rabinically [known as Karmelis]. And you wish to carry from your apartment to a neighboring one, within the building. This is all within a Private Domain [Reshus Hayachid]. Yet to go from one persons private domain to another’s requires an Eruv Chatzeiros. It is questionable if you can choose to make an Eruv with one or some others, or if it is “all or none”.
To carry from your apartment in to the hallway [and not a friends apt.] is a matter of debate, which depends on the halachic definition of the stairwell/hallway. There is room for leniency in this regard.