The English philosopher John Locke, who so strongly influenced the writing of the American constitution, believed so much in liberty that he could not imagine any obligation that could bind you if you didn't personally and individually consent to it. Hence, he had to concoct an elaborate explanation about how all of us really consented to the government. They wrote up some rules for themselves, and agreed to abide by them, and anyone who decided not to join didn't have to, and could just remain in the state of nature. So, we might define a state as the association with the most inclusive membership within a territory. In voluntary associations, rules of membership may be exclusive.
State is the most inclusive association within a territory.
As a generalization, this is a good way to distinguish the state from the less-inclusive associations, such as interest groups and social clubs. But the closer one looks at this characteristic, the more exceptions one finds. It is, however, an exception to our general statement. While these are quirks, and often annoyances in the legal relations among states, they do not pose substantial problems for viewing states as having power over everyone within a territory. Although the state is usually defined territorially, that does not make everyone within the territory a citizen of the state.