From the Wikipedia:
"The definition of public and private domain is related to its relative amount of enclosures, not on strict ownership. It should be noted that this is a particularly complex area of law, as the legal definitions of private and public domains are intricate, although clear. Background knowledge, and definitions, of domain types must be understood before one can fully understand the laws of transference in this context. "
"If the properties enclosed are owned by more than one person, then all the properties must be combined by the acquisition or rental of some right to the properties, and the designation of a meal that is shared by all property owners. The designation of the meal is called an eruv chatzeiros (combining of courtyards) and it is from this that the term eruv is derived.
A community eruv refers to the legal aggregation or "mixture" under Jewish religious property law of separate parcels of property meeting certain requirements into a single parcel held in common by all the holders of the original parcels, which enables Jews who observe the traditional rules concerning Shabbat to carry children and belongings anywhere within the jointly held property without transgressing the prohibition against carrying a burden across a property line on Jewish Sabbath. The legal aggregation is set up to have effect on Shabbat and Yom Kippur only; on all other days, including Yom Tov, ordinary property ownership applies. A valid aggregation has a number of requirements including an agreement among the property-holders and an aggregation ritual." (https://en.wikipedia.org/wiki/Eruv)