There are many businesses that hosts other related or unrelated business as part of their service. Example a plaza/shopping mall will have various stores, owned and operated by others, a university may host other organisations such as book stores, restaurants, computer stores etc. The University of Technology, Jamaica has an organisation/business called the Technology Innovation Centre (TIC), that is a business incubator. TIC itself hosts various businesses not relating to the University. All these hosted businesses uses the address and name of their host as part of their own address.What name is given to these type of businesses that play hosts to other businesses?
One might also refer to a host organization or a support organization. Lesser-used terms (in the context of the question) include octopus (“An organization that has many powerful branches controlled from the centre”), conglomerate (“(business) A corporation formed by the combination of several smaller corporations whose activities are unrelated to the corporation's primary activity”), holding company (“(business) A company whose main purpose is to own shares of other companies as opposed to producing goods and services”), and perhaps shell corporation (sense 26, “(business) A legal entity that has no operations”).
It would depend on the nature of the agreement. If there is shared ownership involved, you may be talking about a parent company. If there is some sort of joint operating agreement, then the organization is probably best termed a "partnering organization." If you have no idea what the relationship is, you might be best of just calling it an "affiliate."